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POWER  DISTRIBUTION  FOR 
ELECTRIC  RAILWAYS 

SECOND  EDITION.     EXTENSIVELY  REVISED 
AND  ENLARGED 


303  Pages.  148  Illustrations.  Price,  $2.50 

IN   PRESS 

ART  OF  ILLUMINATION 


TLATE   I. 


ELECTRIC 
POWER  TRANSMISSION 


A  PRACTICAL    TREATISE  FOR 
PRACTICAL   MEN 


BY 

LOUIS  BELL,  PH.  D. 

MEM.    AM.    INST.    ELEC.    ENG. 


THIRD  EDITION 
REVISED  AND  ENLARGED 


NEW  YORK 
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(INCORPORATED) 


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-  '  0 


-PREFACE   TO   FIRST   EDITION 

THIS  volume  is  designed  to  set  forth  in  the  simplest  possible 
manner  the  fundamental  facts  concerning  present  practice  in 
electrical  power  transmission. 

Busy  men  have  little  time  to  spend  in  discussing  theories  of 
which  the  practical  results  are  known,  or  in  following  the 
derivation  of  formulae  which  no  one  disputes.  The  author  has 
therefore  endeavored,  in  introducing  such  theoretical  consid- 
erations as  are  necessary,  to  explain  them  in  the  most  direct 
way  practicable;  using  proximate  methods  of  proof  when  pre- 
cise and  general  ones  would  lead  to  mathematical  complica- 
tions without  altering  the  conclusion  for  the  purpose  in  hand, 
and  stating  only  the  results  of  investigation  when  the  proc- 
esses are  undesirably  complicated. 

In  writing  of  a  many-sided  and  rapidly  changing  art,  it  is 
impossible  in  a  finite  compass  to  cover  all  the  phases  of  the 
subject  or  to  prophesy  the  modifications  that  time  will  bring 
forth;  hence  the  epoch  of  this  work  is  the  present  and  the 
point  of  view  chosen  is  that  of  the  man,  engineer  or  not,  who 
desires  to  know  what  can  be  accomplished  by  electrical  power 
transmission,  and  by  what  processes  the  work  is  planned  and 
carried  out.  This  treatment  is  not  without  value  to  the  stu- 
dent who  wishes  to  couple  his  investigations  of  electrical 
theory  with  its  application  in  the  hands  of  engineers,  and  puts 
the  facts  regarding  a  very  great  and  important  development  of 
applied  electricity  in  the  possession  of  the  general  reader. 

Such  apparatus  as  is  described  is  intended  to  be  typical  of 
the  methods  used,  rather  than  representative  of  any  particular 
scheme  of  manufacture  or  fashion  in  design.  These  last 
change  almost  from  month  to  month,  while  the  general  con- 
ditions remain  fairly  stable,  and  the  underlying  principles  are 
of  permanent  value. 

January,  1897. 


989189 


PREFACE   TO   SECOND    EDITION. 


SINCE  the  first  edition  of  this  work  was  published  the  changes 
in  the  art  of  electrical  power  transmission  have  been  neither 
great  nor  startling.  There  has  been  steady  and  rapid  growth 
all  along  the  line,  experiments  have  passed  into  quite  common- 
place practice,  and  hopes  have  been  fulfilled  by  events.  The 
most  important  advance  has  been  in  the  matter  of  power 
transmission  at  very  high  voltage  and  to  long  distances.  Two 
years  ago  the  highest  voltage  in  commercial  use  was  14,000,  to- 
day it  is  40,000.  Then  the  longest  transmission  line  was  35 
miles,  now  it  is  80  miles.  This  seems  like  a  long  forward  stride, 
but  there  was  no  serious  doubt  two  years  ago  about  the  practi- 
cability of  either  the  voltage  or  distance  aforesaid,  and  the  way 
had  already  been  mapped  out  by  experiments.  Similarly, 
apparatus  is  on  a  larger  scale  and  in  some  respects  better,  but 
there  have  been  no  material  changes.  Such  change  in  practice 
as  seems  worthy  of  particular  notice  has  been  recorded  in 
Chap.  XIV.,  and  where  experiment  has  advanced  to  certainty 
or  new  light  has  been  thrown  upon  the  art,  the  fact  has  been 
noted  in  the  text.  The  only  considerable  addition  to  our 
knowledge  of  the  theoretical  side  of  the  subject  has  been  in 
the  beautiful  researches  on  high-voltage  phemomena  described 
in  Chap.  XIV.  These  are  not  only  of  great  practical  value,  but 
bring  out  with  wonderful  clearness  the  importance  of  the  elec- 
trical phenomena  which  take  place  outside  of  the  conducting 
wire.  These  have  been  in  full  view  ever  since  the  classical 
paper  of  Poynting  fifteen  years  ago,  but  have  never  before 
been  brought  before  the  eye  of  the  practical  electrician  with 
such  vividness  of  delineation. 

September,  1899. 


PREFACE  TO  THIRD  EDITION. 


IT  is  now  more  than  four  years  since  the  first  edition  of  this 
work  emerged  from  the  press,  and  in  that  period  power  trans- 
mission plants  have  multiplied,  and  methods,  while  affected 
by  no  profound  changes,  have  been  in  so  far  modified  that 
ordinary  revision  is  almost  incapable  of  keeping  the  volume 
within  hailing  distance  of  current  practice.  It  has  therefore 
become  necessary  almost  to  rewrite  some  of  the  chapters, 
adding  considerable  amounts  of  new  matter  and  many  illus- 
trations of  recent  plants  and  apparatus.  A  brief  chapter  on 
commercial  electrical  measurements  has  been  added,  and  a 
final  chapter  on  the  present  state  of  high  voltage  transmission 
has  been  utilized  to  bring  the  latest  developments  of  the  art 
before  the  reader.  The  greatest  changes  of  the  last  two  years 
have  been  in  the  direction  of  the  more  certain  and  efficient 
management  of  very  high  voltages,  enabling  the  engineer  to 
deal  under  commercial  conditions  with  distances  far  greater 
than  had  been  previously  attempted.  Electrical  energy  is  now 
transmitted  successfully  over  distances  closely  approximating 
two  hundred  miles,  and  at  pressures  of  fifty  thousand  to  sixty 
thousand  volts,  although  both  these  extreme  conditions  are 
not  reached  as  yet  in  the  regular  daily  service  of  any  single 
plant.  Each  upward  step  in  working  voltage  has  lessened  the 
uncertainty  that  veils  the  upper  limit  of  commercial  practice, 
and  to-day  the  engineer  who  confines  himself  to  merely  ten 
thousand  volts  in  a  transmission  of  fifteen  or  twenty  miles 
may  fairly  be  regarded  as  ultra-conservative.  There  has  been 
great  improvement  in  subsidiary  apparatus  and  methods  of 
regulation,  and  there  has  grown  up  a  far  fuller  comprehension 
of  alternating  current  methods  than  had  before  been  current. 
The  development  of  a  highly  successful  and  practical  alter- 
nating arc  light  system  is  perhaps  the  greatest  minor  improve- 
ment in  the  art,  relieving  the  engineer  of  one  of  his  most 
troublesome  problems.  Theory  and  practice  are  both  in  a 
State  that  betokens  striking  changes  in  the  near  future. 

December,  1901. 


CONTENTS. 

CHAPTER  PAGE 

I.     ELEMENTARY  PRINCIPLES, i 

II.  GENERAL  CONDITIONS  OF  POWER  TRANSMISSION,          .        .       22 

III.  POWER  TRANSMISSION  BY  CONTINUOUS  CURRENTS,       .        .       75 

IV.  SOME  PROPERTIES  OF  ALTERNATING  CIRCUITS,     .        .        .     122 
V.  POWER  TRANSMISSION  BY  ALTERNATING  CURRENTS,     .         .154 

VI.  SYNCHRONOUS  AND  INDUCTION  MOTORS,        ....     208 

VII.     CURRENT  REORGANIZERS, 266 

VIII.  ENGINES  AND  BOILERS,            .......     290 

IX.     WATER  WHEELS, ;       .     324 

X.     HYDRAULIC  DEVELOPMENT, 361 

XI.  THE  ORGANIZATION  OF  A  POWER  STATION,          .        .        .     388 

XII.     THE  LINE, 417 

XIII.  LINE  CONSTRUCTION,       .        .     \.' 467 

XIV.  CENTRES  OF  DISTRIBUTION,     . 499 

XV.  THE  COMMERCIAL  PROBLEM,           .        .        .        .        .        .557 

XVI.  THE  MEASUREMENT  OF  ELECTRICAL  ENERGY,       .        .        .     574 

XVII.  THE  PRESENT  STATE  OF  HIGH  VOLTAGE  TRANSMISSION,     .     593 


LIST  OF  TABLES. 


PAGE 

EFFICIENCY  OF  WIRE  ROPE  DRIVES,  .  .  .  -37 

WIRE  ROPES  AND  PULLEYS  THEREFOR,  .  .  .  .  .  .41 

Loss  OF  HEAD  IN  HYDRAULIC  PIPES 46 

Loss  OF  AIR  PRESSURE  IN  PIPES, 51 

EFFICIENCIES  OF  ELECTRIC  MOTORS, 61 

EFFICIENCIES  OF  ELECTRIC  AND  OTHER  TRANSMISSIONS,   ...       72 
PERFORMANCE  OF  SMALL  POLYPHASE  MOTORS,  .        .        .        .     258 

STEAM  CONSUMPTION  OF  ENGINES,      .......     299 

EVAPORATIVE  POWER  OF  FUELS,         .        .        .  •  .        .        .     308 

EVAPORATIVE  TESTS  OF  BOILERS,        .        .        .        .        »•       ,  .      .     309 

COAL  CONSUMPTION  OF  ENGINES,  .  .  .  .  .  .  311 

TABLE  FOR  WEIRS, ,  .  ;  .  365 

PROPERTIES  OF  STEEL  HYDRAULIC  PIPE,    .        .        .        .        .        .    380 

SIZE,  RESISTANCE  AND  WEIGHTS  OF  COPPER  WIRES,        .        .        .    420 
PROPERTIES  OF  COPPER  AND  OTHER  WIRES,       .         .        .        9        .431 
IMPEDANCE  FACTORS  OF  COPPER  WIRES,     ......    448 

CAPACITY  AND  INDUCTANCE  OF  LINES,        ......    455 

SIZE,  WEIGHT  AND  TENSILE  STRENGTH  OF  LINE  WIRES,  •*      .         r    470 
SIZES  AND  WEIGHTS  OF  WOODEN  POLES,  .        .        .        .        .     477 

PROPERTIES  OF  DIRECT  AND  ALTERNATING  CURRENT  ARCS,     .        .     510 
COST  OF  COAL  WITH  VARIOUS  ENGINES,    .       . .        .        .        ,        .     558 

COST  OF  POWER  WITH  VARIOUS  ENGINES,  .  .  .'  .  .  560 
COST  OF  ELECTRIC  MOTORS,  .  .  ,  ...  V  •  .  562 

622 
LIST  OF  AMERICAN  TRANSMISSIONS  AT  OR  ABOVE  10,000  VOLTS,  •{  623 

624 


ELECTRIC  TRANSMISSION  OF  POWER. 


CHAPTER  I. 


ELEMENTARY    PRINCIPLES. 


IT  has  long  been  the  fashion  to  speak  of  what  we  are 
pleased  to  call  electricity  as  a  mysterious  "  force,"  and  to 
attribute  to  everything  connected  with  it  occult  characteristics 
better  suited  to  mediaeval  wizardry  than  to  modern  science. 
This  unhappy  condition  of  affairs  has,  in  the  main,  come  about 
through  the  indistinctness  of  some  of  our  fundamental  ideas 
and  inexactitude  in  expressing  them. 

To  speak  specifically,  there  has  been,  even  in  the  minds 
and  writings  of  some  who  ought  to  know  better,  a  tendency 
toward  confusing  the  extremely  hazy  individuality  of  "  elec- 
tricity "  with  the  sharply  defined  properties  of  electrical 
energy.  We  have  been  so  overrun  by  theories  of  electricity, 
two-fluid,  one-fluid,  and  non-fluid, — by  electrically  ''charged" 
atoms  and  duplex  ethers,  that  we  have  well-nigh  forgotten  the 
very  great  uncertainty  as  to  the  concrete  existence  of  elec- 
tricity itself.  Even  admitting  it  to  be  an  entity,  it  most 
assuredly  is  not  a  force,  mysterious  or  otherwise.  Electrical 
force  there  is,  and  electrical  energy  there  is,  and  with  them 
we  can  freely  experiment,  but  for  most  practical  purposes 
"electricity"  is  merely  the  factor  connecting  the  two.  It  is 
related  to  electrical  energy  just  as  that  other  hypothetical 
fluid  "caloric  "  was  supposed  to  be  related  to  heat  energy. 
The  analogy  is  not  absolutely  exact,  but  it  nevertheless  sum- 
marizes the  real  facts  in  the  case. 

The  day  has  passed  wherein  we  were  at  liberty  to  think  of 
"  electricity  "  as  flowing  through  a  material  tube  or  as  plas- 
tered upon  bodies  like  a  coat  of  paint.  The  things  with  which 


2  ELECTRIC    TRANSMISSION   OF  POWER. 

we  have  now  to  deal  are  the  various  factors  of  electrical 
energy. 

It  is  the  purpose  of  this  chapter  to  treat  of  that  form  of 
energy  which  we  denominate  electrical,  to  discuss  its  relation 
to  other  forms  of  energy  and  some  of  the  transformations 
which  they  may  reciprocally  undergo. 

Speaking  broadly,  energy  is  power  of  doing  work.  The 
energy  of  a  body  at  any  moment  represents  its  inherent  capacity 
for  doing  work  of  some  sort  on  other  bodies.  This,  however, 
must  not  be  understood  as  implying  that  the  aforesaid  energy 
^  cc'  $s  limited  by]  our  power  of  utilizing  it.  We  may  or  may  not  be 
able  to  employ  it  to  advantage  or  under  possible  conditions. 
As  an  t;>  ample  take  the  massive  weight  of  a  pile  driver. 
Raised  to  its  full  height  it  possesses  a  certain  amount  of 
gravitational  energy — a  possibility  of  doing  useful  work.  This 
energy  is  temporarily  unemployed  and  appears  only  as  a  stress 
on  the  supporting  rope  and  frame-work.  Under  these  cir- 
cumstances, wherein  the  energy  exists  in  static  form,  it  is- 
generally  known  as  potential  energy. 

Now  let  the  weight  fall  and  with  swiftly  gathering  velocity 
it  strikes  the  pile  and  does  work  upon  it,  settling  it  deep  into 
the  mud.  The  energy  due  to  the  blow  of  the  moving  weight, 
energy  of  motion  in  other  words,  is  called  kinetic.  But  at  the 
bottom  of  its  fall  the  weight  still  has  potential  energy  with 
reference  to  points  below  it,  and  we  realize  this  as  the  pile 
settles  lower  and  each  successive  blow  becomes  more  forceful. 
At  some  point  we  are  unable  further  to  utilize  the  fall,  and 
have  then  reached  the  limit  of  the  available  energy  in  this  par- 
ticular case. 

We  must  not  forget,  however,  that  each  time  the  weight 
was  lifted,  work  had  to  be  done  against  gravitation  to  give  the 
weight  its  point  of  vantage  with  respect  to  available  energy. 
This  work  was  probably  done  by  utilizing  the  energy  of 
expanding  steam — in  other  words,  the  energy  of  the  steam 
was  transformed  through  doing  work  on  the  piston  inta 
kinetic  energy  of  the  latter,  which,  through  doing  work  against 
gravitation,  has  been  enabled  again  to  reappear  as  the  energy 
of  a  falling  body,  and  to  do  work  on  the  driven  pile.  And 
back  of  the  steam  energy  is  the  heat  energy,  by  which  work  is 
done  on  the  water  in  the  boiler,  and  yet  back  of  this  the 


ELEMENTARY   PRINCIPLES.  3 

chemical  energy  of  the  coal,  transformed  into  heat  energy  and 
doing  work  on  the  minute  particles  of  iron  in  the  boiler,  for 
we  know  that  heat  is  a  species  of  kinetic  energy. 

Even  the  work  done  on  our  pile  is  not  permitted  to  go  un- 
transformed  into  energy.  Part  is  transformed  into  heat  energy 
through  friction  and  compression  of  the  pile,  part  through  fric- 
tion of  the  water,  and  part  raises  ripples  that  may  lift  against 
gravity  chips  and  pebbles  on  a  neighboring  shore.  Other  frac- 
tions go  into  the  vibrational  energy  of  sound;  into  heating  the 
weight  so  that  it  gives  out  warmth — radiant  energy — to  the  hand 
when  held  near  it  and  to  the  surrounding  air;  and  into  electri- 
cal work  done  on  the  weight  and  neighboring  objects,  for  the 
weight  unquestionably  receives  a  minute  amount  of  electrical 
energy  at  each  blow.  Thus  a  comparatively  simple  mechanical 
process  involves  a  long  series  of  transformations  of  energy. 

No  energy  is  ever  created  or  destroyed,  it  merely  is  changed 
in  form  to  reappear  elsewhere,  and  work  done  is  the  link 
between  one  form  of  energy  and  another.  And  we  may  lay 
down  another  law  of  almost  as  serious  import:  No  form  of 
energy  is  ever  transformed  completely  into  any  other. 

On  the  contrary  the  general  rule  is  that  with  each  transfor- 
mation several  kinds  of  energy  appear  in  varying  amounts,  and 
among  them  we  may  always  reckon  heat.  The  object  of  any 
transformation  is  usually  a  single  form  of  energy,  hence  practi- 
cally no  such  thing  as  perfectly  efficient  transformation  can  be 
obtained.  The  energy  by-products  for  the  most  part  cannot 
be  utilized  and  are  frittered  away  in  useless  work  or  in  storing 
up  kinds  of  potential  energy  that  cannot  be  employed. 

The  greatest  loss  is  in  heat,  which  is  dissipated  in  various 
ways  and  cannot  be  recovered.  The  presence  of  unutilized 
heat  always  denotes  waste  of  energy. 

From  what  has  gone  before  we  can  readily  appreciate  that 
when  we  do  work  with  the  object  of  rendering  available  a 
particular  kind  of  energy,  the  method  must  be  intelligently 
selected,  else  there  will  result  useless  by-products  of  energy 
which  will  seriously  lower  the  efficiency  of  the  operation. 

Whenever  possible  we  utilize  potential  energy  already  exist- 
ing in  securing  a  transformation.  Thus  if  heat  is  wanted,  the 
easiest  way  of  getting  it  is  to  burn  coal,  and  to  allow  its  energy 
to  become  kinetic  as  heat.  If  we  want  mechanical  work  done, 


4  ELECTRIC   TRANSMISSION  OF  POWER. 

we  set  heat  energy  to  work  in  the  most  efficient  way  practi- 
cable. If  electrical  energy  is  desired  we  set  the  energy  of 
steam  to  revolving  the  armature  of  a  dynamo.  If  the  right 
method  of  transformation  is  not  chosen,  much  of  the  energy 
will  turn  up  in  forms  that  we  do  not  want  or  cannot  utilize. 
Burning  coal  is  a  very  bad  way  of  getting  sound,  just  as  play- 
ing a  cornet  is  but  a  poor  means  of  getting  heat,  although  a 
fire  does  produce  a  trifling  amount  of  sound,  and  a  cornet  by 
continual  vibration  must  be  warmed  to  a  minute  degree. 

These  seem,  and  perhaps  are,  extreme  instances,  but  when 
we  realize  that,  somewhat  to  the  discredit  of  human  ingenuity, 
less  than  one-twentieth  of  the  electrical  energy  supplied  to  an 
incandescent  lamp  appears  in  the  form  of  light,  the  comparison 
becomes  grimly  suggestive. 

Understanding  now  that  in  order  to  obtain  energy  in  any 
given  form  (such  as  electrical)  particular  methods  of  transfor- 
mation must  be  used  in  order  to  secure  anything  like  efficiency, 
we  may  look  a  little  more  closely  at  various  types  of  energy  to 
discover  the  characteristics  that  may  indicate  efficient  methods 
of  transformation,  particularly  as  regards  electrical  energy. 

Speaking  broadly,  one  may  divide  energy  into  three  classes: 

ist.  Those  forms  of  energy  which  have  to  do  with  move- 
ments of,  or  strains  in,  masses  of  matter.  In  this  class  may 
be  included  the  ordinary  forms  of  kinetic  energy  of  moving 
bodies  and  the  like. 

2d.  Those  which  are  concerned  with  movements  of,  or 
strains  in,  the  molecules  and  atoms  of  which  material  bodies 
are  composed.  In  this  class  we  may  reckon  heat,  latent  and 
specific  heats,  energy  of  gases  and  perhaps  chemical  energy. 

3d.  All  forms  of  energy  which  have  to  do  with  strains  which 
can  exist  outside  of  ordinary  matter,  *.  e.,  every  kind  of 
radiant  energy  and  presumably  electrical  energy. 

These  classes  are  not  absolutely  distinct;  for  example,  we 
do  not  know  the  relation  of  chemical  energy  to  the  third  class, 
nor  of  gravitational  energy  to  any  class,  but  such  a  division 
serves  to  keep  clearly  in  our  minds  the  kind  of  actions  to 
which  our  attention  is  to  be  directed. 

It  is  only  within  the  past  few  years  that  we  have  been  able 
with  any  certainty  to  classify  electrical  energy,  and  even  now 
much  remains  to  be  learned.  For  a  very  long  while  it  has 


ELEMENTARY  PRINCIPLES.  5 

been  known  that  light,  /.  e.,  luminous  energy,  must  be  propa- 
gated through  a  medium  quite  distinct  from  ordinary  matter 
and  possessing  certain  remarkable  properties.  It  was  well 
known  that  luminous  energy  is  transferred  through  this 
medium  by  vibratory  or  wave  motion.  Even  the  period  of  the 
vibrations  and  the  lengths  of  the  waves  were  accurately  meas- 
ured, and  from  these  and  similar  measurements  it  has  been 
possible  to  classify  the  mechanical  properties  of  this  medium, 
universally  called  "the  ether,"  until  we  really  know  more 
about  them  than  about  the  properties  of  many  kinds  of 
ordinary  matter — a  number  of  the  rare  metals  for  example. 

The  next  important  step  was  the  discovery,  verified  in  the 
most  thorough  manner,  that  what  had  been  known  as  radiant 
heat,  such  as  we  get  from  the  sun  or  any  very  hot  body,  is 
really  energy  of  the  same  kind  as  light.  That  is,  it  was  found 
to  be  energy  of  wave  motion  of  precisely  the  same  character 
and  in  the  same  medium,  differing  only  in  frequency  and  wave 
length.  It  also  has  turned  out  in  similar  fashion  that  what 
had  been  called  "actinic  "  rays,  that  are  active  in  attacking  a 
photographic  plate  and  producing  some  other  kinds  of  chemi- 
cal action,  are  only  light  rays  of  shorter  wave  length  than 
usual,  and  so  ordinarily  invisible  to  the  eye. 

So  much  having  been  ascertained  it  became  clear  that 
instead  of  three  kinds  of  energy — "heat,  light  and  actinism," 
we  were  really  dealing  with  only  one — radiant  energy,  vibrat- 
ing energy  in  the  ether,  varying  in  effect  as  it  varies  in  fre- 
quency. Speaking  in  an  approximate  way,  such  wave  energy 
has  a  frequency  of  six  hundred  thousand  billion  vibrations  per 
second  and  a  velocity  of  propagation  of  about  a  hundred  and 
eighty-five  thousand  miles  per  second,  so  that  each  wave  is 
not  far  from  one  fifty-thousandth  of  an  inch  long.  These 
dimensions  are  true  of  light  waves;  chemical  action  can  be 
produced  by  waves  of  half  the  length,  while  so  called  heat 
rays  may  be  composed  of  waves  two  or  three  times  as  long  as 
those  of  light.  Such  figures  are  startling,  but  they  can  be 
verified  with  an  accuracy  greater  than  that  of  ordinary 
mechanical  measurements. 

We  see  that  this  radiant  energy  is  capable  of  producing 
various  disturbances  perceptible  to  our  senses,  such  as  chemi- 
cal action,  light  and  heat,  and  that  these  different  effects 


6  ELECTRIC   TRANSMISSION  OF  POWER. 

simply  correspond  to  waves  of  energy  having  different  fre- 
quencies and  wave  lengths.  This  being  so,  it  is  not  unnatural 
to  suppose  that  at  still  different  frequencies  other  effects 
might  be  noted.  This  idea  gains  further  probability  from  the 
experimental  fact  that  waves  of  very  different  frequencies, 
traverse  the  ether  with  precisely  the  same  velocity,  showing  no 
signs  of  slowing  down  or  dying  out,  so  that  there  seems  to  be 
no  natural  limit  to  their  length. 

During  the  past  half  dozen  years  it  has  been  clearly  shown 
that  "radiant  energy"  is  capable  of  producing  profound 
electrical  disturbances,  such  as  violent  oscillations  of  electrical 
energy  in  conducting  bodies,  and  that  these  effects  exist  what- 
ev.er  the  frequency  of  the  ether  waves  concerned.  This  very 
important  fact  was  clearly  foreseen  by  Maxwell  more  than, 
twenty  years  ago,  regarding  light,  and  his  prediction  has  been 
thoroughly  verified  through  the  persistent  researches  of  the 
late  Professor  Hertz  and  others. 

This  discovery  is  often  expressed  by  saying  that  radiant 
energy  is  an  electro-magnetic  disturbance,  or  that  light  is  one 
kind  of  electrical  action.  It  is  more  strictly  accurate  to  say 
that  radiant  energy,  just  as  it  produces  chemical  disturbances, 
on  the  photographic  plate,  affects  the  eye  as  light,  and  material 
bodies  as  heat,  is  also  capable  of  producing  electrical  effects 
when  transferred  to  the  proper  media.  Most  of  our  experi- 
ments on  its  electrical  effects  have  been  performed  with  waves 
many  thousand  times  longer  than  those  of  light,  but  their  gen- 
eral character  has  proved  to  be  exactly  the  same. 

A  given  substance  may  be  differently  related  to  waves  of 
radiant  energy  of  different  lengths,  but  the  phenomena  are 
still  essentially  the  same.  For  instance,  a  plate  of  hard 
rubber  is  thoroughly  opaque  to  waves  of  a  length  correspond- 
ing to  light,  but  is  quite  transparent  to  those  of  considerably 
greater  length,  such  as  can  produce  thermal  or  electrical 
effects.  A  plate  of  alum  will  let  through  light  waves  and  very 
long  waves,  but  will  stop  most  of  those  which  are  efficient  in 
producing  heat.  A  thick  sheet  of  metal  is  quite  opaque  to  all 
known  waves  of  radiant  energy.  Hence  the  fact  noted  long 
ago  by  Maxwell,  that  all  good  conductors  are  opaque  to  light, 
although  the  converse  is  not  true. 

The  substance  of  all  this  is,  that  the  same  sort  of  disturb- 


ELEMENTARY  PRINCIPLES.  7 

ance  in  the  ether  which  produces  light  is  also  competent  to  set 
up  electrical  actions  in  material  bodies,  and  conversely,  such 
actions  may  and  do  produce  corresponding  disturbances  in  the 
ether,  which  are  thus  transferred  to  other  bodies.  Such  a 
transference  corresponds  to  all  that  we  know  concerning  the 
velocity  with  which  electrical  and  electro-magnetic  disturb- 
ances pass  from  body  to  body.  It  is  equally  certain  that  this 
velocity  totally  transcends  anything  we  could  hope  to  obtain 
from  bodies  having  the  dynamical  properties  of  ordinary 
matter,  while  it  does  fit  exactly  the  dynamical  properties  of 
the  ether. 

We  are  thus  forced  to  the  conclusion  that  when  an  electrical 
current,  as  we  say,  "passes  along  "a  wire,  whatever  a  "  cur- 
rent "  may  be,  it  is  not  simply  transferred  from  molecule  to 
molecule  in  the  wire  as  sound  or  heat  would  be,  but  that  there 
is  an  immensely  rapid  transfer  of  energy  in  the  neighboring 
ether  that  reaches  all  points  of  the  wire  almost  simultaneously. 
It  takes  a  measurable  time  for  the  electrical  energy  to  reach 
and  utilize  the  centre  of  the  wire,  although  its  progress  along  the 
surface,  thanks  to  the  free  ether  outside,  is  enormously  rapid. 

Thus  takes  place  what  is  generally  called  a  "flow  of  elec- 
tricity "  along  the  wire.  Looking  at  the  process  more  closely, 
the  nearest  approach  to  flow  is  the  transfer  of  energy  along 
the  wire  by  means  of  stresses  in  the  ether  which  in  turn  set  up 
strains  in  the  matter  along  their  course. 

Whenever  we  cause  in  matter  the  particular  stress  which  we 
call  electromotive  force  for  lack  of  a  more  exact  name,  the 
resulting  strain  is  electrification,  and  if  the  stress  be  applied 
at  one  point  of  a  conducting  body,  the  strain  is  immediately 
transferred  to  other  points  by  the  stresses  and  strains  in  the 
surrounding  ether.  Wherever  this  transference  of  strain  exists 
we  have  an  electrical  current,  although  this  name  is  generally 
reserved  for  those  cases  in  which  there  exists  a  perceptible 
transference  of  energy  by  the  means  aforesaid.  If  the  condi- 
tions are  such  that  energy  must  be  steadily  supplied  to  keep 
up  the  electromotive  stress  we  have  such  a  state  of  things  as 
we  find  in  a  closed  circuit  containing  a  battery. 

To  cause  such  a  flow  of  energy  we  must  first  find  means  of 
setting  up  electromotive  stress  capable  of  being  propagated 
through  the  ether.  Now  atoms  and  molecules  are  the  only 


8  ELECTRIC   TRANSMISSION  OF  POWER. 

handles  by  which  we  can  get  hold  of  the  ether.  Only  in  so  far 
as  we  can  work  through  them  can  we  do  work  on  the  ether. 

As  a  matter  of  fact  we  cannot  do  work  of  any  kind  on  the 
molecules  of  a  body  without  setting  up  electrical  stresses  of 
some  sort.  In  most  cases  of  mechanical  work,  which  in  the 
main  produces  stress  on  the  molecules  only  by  strains  in 
the  mass,  the  energy  appears  mainly  as  heat,  and  is  only  inci- 
dentally electrical,  as  for  instance  the  energy  wasted  in  a 
heated  journal. 

When,  however,  by  any  device  we  do  work  more  directly  on 
the  molecules  of  a  body,  or  on  the  atoms  which  compose  the 
molecules,  we  are  more  than  likely  to  transform  much  of  this 
work  into  electrical  energy.  As  a  rough  example  of  the  two 
kinds  of  action  just  mentioned,  pounding  a  body  heats  it  with- 
out causing  any  considerable  electrification,  while  on  the  other 
hand  rubbing  it  rather  gently,  sets  up  a  considerable  electrifica- 
tion without  heating  it  noticeably. 

In  fact,  for  many  centuries,  friction  was  the  only  known 
method  of  causing  electrification.  Later,  as  is  well  known,  it 
was  discovered  that  certain  sorts  of  chemical  action,  which  has 
to  do  directly  with  interchanges  of  energy  between  molecules, 
were  very  potent  in  electrical  effects.  With  this  discovery 
came  the  ability  to  deal  with  steady  transfers  of  electrical 
energy  in  considerable  amount  (electric  currents),  instead  of 
the  relatively  slight  and  transitory  effects  previously  known, 
(electrification,  "  frictional"  electricity). 

To  clear  up  the  real  nature  of  this  difference  it  is  well  to 
consider  what  we  mean  by  saying  that  a  body  is  electrified,  or 
has  an  electrical  charge.  In  other  words,  what  is  electrifica- 
tion ?  Not  very  many  years  ago  this  question  would  have 
been  answered  by  saying  that  a  quantity  of  a  substance,  posi- 
tive electricity  (or  negative  as  the  case  might  be),  had  been 
communicated  to  the  body  in  question;  that  this  remarkable 
substance  could  reside  only  at  the  surface  of  the  body  and 
was  able  to  produce  in  surrounding  bodies  exactly  an  equal 
quantity  of  negative  electricity;  that  this  "charge  "of  elec- 
tricity would  repel  another  "charge"  of  the  same  substance 
placed  near  it,  or  attract  a  charge  of  its  opposite,  the  other 
substance  called  negative  electricity;  and  much  more  to  the 
same  effect.  All  this  was  a  very  convenient  hypothesis — it 


ELEMENTARY  PRINCIPLES.  9 

explained,  after  a  fashion,  the  common  facts  and  enabled 
investigators  to  discover  many  important  electrical  relations 
and  laws.  But  it  expressed  much  more  than  there  was  any 
reason  to  know.  From  the  standpoint  of  our  modern  doc- 
trines of  energy  electrification  is  a  very  different  thing. 

Let  an  electromotive  stress  (from  whatever  source)  be 
applied  to  a  body,  a  metallic  sphere  for  example,  long  enough 
to  transfer  to  it  a  finite  amount  of  energy.  This  energy 
appears  as  stresses  and  strains  in  the  ether  everywhere  about 
the  body  under  consideration  and  thence  extends  to  the  mole- 
cules and  atoms  of  neighboring  bodies,  causing  "  induced 
charges."  It  is  as  if  one  were  to  fill  a  box  with  jelly,  and  then 
pull  or  push  or  twist  a  rod  embedded  in  its  centre.  The 
result  would  be  strains  in  the  rod,  the  jelly  and  the  box,  and  in 
a  general  way  the  total  stress  on  the  box  would  equal  that  on 
the  rod.  By  proper  means  we  could  detect  the  strain  all 
through  the  substance  of  the  jelly,  but  most  easily  by  its  varia- 
tions from  place  to  place. 

We  do  not  know  exactly  what  sort  of  a  strain  in  our  ether 
jelly  is  produced  by  electromotive  stress,  but  we  do  know  that 
it  possesses  the  quality  of  endedness,  so  that  the  strains  in 
the  matter  concerned,  /.  e.,  in  the  ball  and  surrounding  bodies, 
are  equal  and  opposite. 

In  fact  the  two  "  charges  "  are  merely  the  two  ends  of  the 
same  strain  in  the  ether.  They  appear  to  us  to  be  real  attri- 
butes of  the  two  opposed  surfaces,  because  at  these  surfaces 
the  dynamical  constants,  such  as  density,  elasticity,  etc.,  of 
the  medium  through  which  the  strain  is  propagated,  change 
in  value,  and  differences  in  state  of  strain  there  become  plainly 
manifest. 

In  electric  currents  we  have  a  very  different  state  of  things. 
The  energy  supplied  by  the  electromotive  stress,  instead  of 
becoming  potential  as  electrostatic  strain,  and  producing 
"charge,"  does  work  and  is  transformed  into  other  kinds  of 
energy,  thermal  or  chemical,  mechanical  or  luminous. 

When  a  stress  of  whatever  kind  is  applied  to  a  body,  only  a 
limited  amount  of  energy  can  be  transferred  by  it  so  long  as 
the  energy  remains  potential.  Thus  in  our  box  of  jelly  before 
referred  to,  a  twist  of  given  intensity  applied  to  the  stick,  as 
for  instance  by  a  string  wound  around  it  and  pulled  by  a  given. 


io  ELECTRIC   TRANSMISSION-  OF  POWER. 

weight,  can  only  transfer  energy  until  the  stresses  produced  in 
the  jelly  come  to  an  equilibrium  with  it.  On  the  other  hand  if 
the  box  were  filled  with  water  and  the  stick  were  the  axle  of  a 
sort  of  paddle  wheel,  the  very  same  intensity  of  twist  could  go 
on  communicating  energy  to  the  water  as  long  as  one  chose 
to  apply  the  necessary  work. 

This  roughly  expresses  the  difference  between  electric 
charge  and  electric  current,  viewed  from  the  standpoint  of 
energy.  An  electromotive  stress  applied  to  a  wire  charges  it 
and  then  the  transfer  of  energy  ceases.  If  the  same  stress  be 
applied  under  conditions  that  allow  work  to  be  done  by  it, 
energy  will  be  transferred  so  long  as  the  stress  is  kept  up.  In 
an  open  electric  circuit  we  have  a  charge  as  the  result  of 
electromotive  stress.  When  the  circuit  is  closed,  /.  e.,  when  a 
continuous  medium  is  furnished  on  which  work  can  be  done, 
we  have  an  electric  current.  The  amount  of  this  work  and 
the  flow  of  electrical  energy  that  produces  it  depends  on  the 
nature  of  the  circuit.  Certain  substances,  especially  the 
metals,  and  of  metals  notably  copper  and  silver,  permit  a 
ready  continuous  transfer  of  energy  in  and  about  them. 
Such  substances  are  called  good  conductors.  The  real  trans- 
fer of  energy  takes  place  ultimately  via  the  ether,  but  its 
amount  is  limited  by  the  amount  and  character  of  the  matter 
through  which  work  can  be  done. 

Whenever  the  strains  in  the  ether,  such  as  we  recognize  in 
connection  with  electrical  charge,  shift  through  space  as  when 
a  current  is  flowing,  other  strains  bearing  a  certain  rela- 
tion to  the  direction  of  flow  are  made  manifest.  Where  there 
is  a  rapid  and  intense  flow  of  energy  these  strains  are  very 
great  and  important  compared  with  any  electrostatic  strains 
that  exist  outside  the  conducting  circuit.  In  other  cases  they  may 
be  quite  insignificant.  These  strains  are  electro-magnetic,  and 
with  them  we  have  to  do  almost  exclusively  in  practical  elec- 
trical engineering.  They  appear  wherever  there  is  a  moving 
electrical  strain,  whether  produced' by  moving  a  charged  body 
or  causing  the  charge  upon  a  body  to  move. 

Both  kinds  of  strains  exist  in  radiant  energy,  as  in  other 
cases  of  flowing  energy.  The  stresses  in  electro-magnetic 
energy  are  at  right  angles  both  to  the  electrostatic  stresses 
and  to  the  direction  of  their  motion  or  flow.  If  for  example 


ELEMENTARY  PRINCIPLES.  II 

we  have  a  flow  of  electrical  energy  in  a  straight  wire  (Fig.  i), 
the  electro-magnetic  stresses  are  in  circles  about  it. 

If  A  be  a  wire  in  which  the  flow  of  energy  is  straight  down 
into  the  paper  the  electro-magnetic  stresses  are  in  circles  in 
the  direction  shown  by  the  arrow  heads.  If  the  wire  be  bent 
into  a  ring  (Fig.  2),  with  the  current  flowing  in  the  direction  of 
the  arrows,  then  the  electro-magnetic  stresses  will  be  (follow- 
ing Fig.  i)  in  such  direction  as  to  pass  downward  through  the 
paper  inside  the  ring. 

These  electro-magnetic  stresses  constitute  what  we  call  a 
magnetic  field  outside  the  wire.  The  intensity  of  this  field  can 
be  increased  by  increasing  the  flow  of  energy  in  the  desired 


FIG.  i.  FIG.  2. 

region  in  the  systematic  way  suggested  by  Fig.  2.  If,  for 
•example,  we  join  a  number  of  rings  like  Fig.  2  into  a  spiral 
coil  shown  in  section  in  Fig.  3,  in  which  the  current  flows 
downward  into  the  paper  in  the  lower  edge  of  the  spiral, 
there  will  be 'produced  a  magnetic  field  in  which  the  stresses 
have  the  direction  shown  by  the  arrows.  Such  a  spiral  consti- 
tutes a  genuine  magnet,  and  if  suspended  so  as  to  be  free 
to  move  would  take  up  a  north  and  south  position  with  its  right- 
hand  end  toward  the  north.  In  and  about  the  spiral  there 
exists  a  magnetic  "field  of  force,"  which  is  merely  another 
way  of  saying  that  the  ether  there  is  under  electro-magnetic 
stress.  Its  condition  of  strain  is  closely  analogous  to  that 
about  an  electrified  body,  and,  as  in  that  case,  there  is  no 
work  done  on  the  ether  after  the  strains  are  once  established, 
since  the  energy  then  becomes  potential.  While  this  is  being 
accomplished  work  is  done  just  as  when  a  body  is  charged. 


12 


ELECTRIC   TRANSMISSION  OF  POWER. 


If  now  setting  up  such  an  electro-magnetic  field  requires 
energy  to  be  spent  by  causing  a  current  to  flow  in  the  spiral, 
we  should  naturally  expect  that  if  the  same  field  could  be  set 
up  by  extraneous  means,  energy  would  momentarily  be  spent 
on  the  spiral  in  producing  stresses  and  strains  similar  to  those 


FIG.  3. 


that  set  up  the  original  field.     This  is  found  to  be  so,  the 
process  working  backward  as  well  as  forward. 

If,  for  example,  we  have  two  rings  (Fig.  4),  and  by  sending 
a  current  around  one,  transfer  energy  to  the  medium  outside 
it,  this  energy  will  set  up  an  electromotive  stress  in  the  other 


FIG.  4. 

ring.  The  direction  of  this  stress  is  not  at  once  obvious,  but 
we  can  get  a  very  clear  idea  of  it  by  considering  the  work 
done.  If  current  is  started  in  A  (Fig.  4),  in  the  direction 
shown,  electro-magnetic  stresses  are  produced  in  the  direction 
of  the  arrow  C.  If  these  are  to  do  work  on  B,  the  electro- 
motive stress  in  the  latter  cannot  have  such  a  direction  as  to 
set  up  on  its  own  account  a  magnetic  field  that  would  assist 


ELEMENTARY  PRINCIPLES.  i$ 

that  of  A,  otherwise  we  could  increase  the  field  indefinitely 
without  added  expenditure  of  energy.  Therefore  the  electro- 
motive stress  in  B,  and  hence  the  current,  must  be  in  a  direc- 
tion opposing  the  original  current  in  A,  as  shown  in  the  figure. 

In  like  manner  if  the  current  in  A  be  stopped  and  the  field 
due  to  it  therefore  changes,  there  are  changes  in  the  electro- 
magnetic stresses  about  B,  that  again  set  up  an  electromotive 
stress  in  it.  If,  however,  this  change  of  stress  is  to  do  work, 
the  electromotive  stress  in  B  must  be  of  such  direction  as  to 
oppose  by  its  field  the  change  in  the  field  of  A — /'.  e.,  it  must 
change  its  direction  and  will  now  give  us  a  current  in  the 
same  direction  as  the  original  one  in  A.  All  this  follows  the 
general  law,  that  if  work  is  to  be  done  by  any  stress  it  must 
be  against  some  other  stress.  There  can  be  no  work  without 
resistance. 

In  Fig.  4  we  have  the  fundamental  facts  of  current  induction 
on  which  depend  most  of  our  modern  methods  of  generating 
and  working  with  electrical  energy.  Summed  up  they  amount 
to  saying  that  whenever  there  is  a  change  in  the  electro- 
magnetic stresses  about  a  conductor,  work  is  done  upon  it, 
depending  in  direction  and  magnitude  on  the  direction  and 
magnitude  of  the  change  in  the  stresses. 

This  is  equally  true  whether  the  stresses  change  in  absolute- 
value  or  whether  the  conductor  changes  its  relation  to  them. 
Thus  in  Fig.  4,  if  A  carries  an  electrical  current  the  result  on 
B  is  the  same  whether  the  field  of  A  changes  through  cessation 
of  the  current,  or  whether  the  same  change  in  the  stresses, 
about  B  is  produced  by  suddenly  pulling  B  away  from  A.  The 
rate  at  which  work  is  done  depends  on  the  rate  at  which  the 
stresses  are  caused  to  change,  as  might  be  expected.  So  long 
as  the  stresses  are  constant  with  reference  to  the  conductor  in 
which  current  is  to  be  induced,  no  work  can  be  done  upon  it. 

These  principles  form  the  foundation  of  the  dynamo,  motor, 
alternating  current  transformer  and  many  other  sorts  of  elec- 
trical apparatus.  Their  details  may  differ  very  widely,  but  we 
can  get  all  the  fundamental  ideas  from  a  consideration  of  Figs. 
3  and  4.  To  define  somewhat  the  specific  idea  of  the  dynamo, 
consider  what  happens  when  a  conducting  wire  is  thrust  into 
a  magnetic  field  such  as  is  produced  by  a  coil,  as  in  Fig.  5. 
As  in  Fig.  3,  let  the  current  in  the  coil  be  flowing  down- 


14  ELECTRIC   TRANSMISSION   OF  POWER. 

ward  into  the  paper  in  the  lower  half  of  the  figure.  A  is  a 
wire  perpendicular  to  the  plane  of  the  paper  in  front  of  the 
coil,  its  ends  being  united  at  any  distant  point  that  is  con- 
venient. Knowing  that  moving  the  wire  into  the  field  will  set 
up  electromotive  stresses  in  it,  we  can  as  before  determine 


FIG.  5. 

their  direction  by  remembering  that  work  must  be  done. 
That  is  (see  Fig.  i),  the  induced  current  will  flow  through  A 
downward  into  the  paper.  In  passing  out  of  the  field  the  cur- 
rent would  be  upward. 

We  have  so  far  neglected  the  rest  of  the  circuit.  To  be 
exact  we  should  consider  it  as  in  Fig.  6.  Following  the  same 
line  of  reasoning  as  in  Fig.  5,  we  see  that  while  the  ring  A  is 
entering  the  magnetic  field  the  current  induced  in  it  must  be 
opposite  to  that  in  the  inducing  coil  (see  Fig.  4).  When  the 


coil  is  leaving  the  field,  however,  this  direction  will  be 
reversed.  Considering  the  coil  A  as  a  whole,  we  see  that  so 
long  as  the  total  field  tending  to  set  up  stresses  in  it  is  increas- 
ing, a  current  will  be  induced  opposed  to  that  in  the  inducing 
coil.  While  the  total  field  is  diminishing,  the  induced  current 
will  be  in  the  other  direction.  The  work  that  is  spent  in 


ELEMENTARY  PRINCIPLES.  15 

moving  the  coil  A  will  for  the  most  part  reappear  as  electrical 
energy  in  that  coil.  Arrange  the  parts  of  Fig.  6,  so  that  the 
motion  of  A  can  be  accomplished  uniformly  and  continuously 
and  we  should  have  a  true,  though  rudimentary,  dynamo. 
Such  a  structure  could  be  made  by  fixing  A  to  the  end  of  an 
arm  pivoted  at  the  other  end  and  then  revolving  the  arm  so 
that  at  each  revolution  the  coil  A  would  sweep  through  the 
field  of  the  magnetizing  coil  (see  Fig.  7).  The  result  of  this, 
as  we  have  seen,  would  be  on  entering  the  field  a  current  in 
one  direction,  and  on  leaving,  a  current  in  the  other.  There 


would  thus  be  an  alternating  current  developed  in  the  ring  A. 
If  it  were  cut  at  some  point  and  wires  led  down  the  arm  and 
to  two  metal  rings  on  the  axis  B  we  could  obtain,  by  pressing 
brushes  on  these  rings,  an  alternating  current  in  any  outside 
circuit.  To  make  more  of  the  revolution  of  the  arm  useful 
we  could  arrange  inducing  coils  in  a  circle  about  B.  There 
would  then  be  an  alternation  as  A  passed  each  coil. 

All  these  devices,  however,  would  produce  comparatively 
weak  effects,  because  it  is  difficult  to  produce  powerful  magnetic 
stresses  in  so  simple  a  way.  There  are  very  few  materials  in 
which  magnetic  stresses  are  easily  set  up  or  propagated. 
Chief  among  these  is  iron,  which  bears  somewhat  the  same  rela- 
tion to  magnetic  actions  that  copper  does  to  electrical  ones. 
By  giving  to  the  coil  in  Fig.  7  a  core  of  soft  iron  the  electro- 
magnetic effects  obtained  from  it  would  be  greatly  enhanced. 
They  are  comparatively  feeble  in  air  and  the  more  iron  we  put 
in  their  path  the  better.  Developing  this  idea  we  have  in 
Fig.  8  a  much  better  device  for  setting  up  electric  currents. 
Here  the  coil  of  Fig.  7  is  wound  around  an  iron  core  the  ends 
of  which  are  brought  near  together.  The  arm  of  Fig.  7  is 
also  of  iron  with  enlarged  ends  and  the  ring  A  is  replaced  by 
a  coil  of  several  turns. 


16  ELECTRIC    TRANSMISSION  OF  POWER. 

The  magnetic  stresses  brought  to  bear  on  the  coil  A  are  thus 
made  comparatively  powerful.  Following  out  on  Fig.  8  the 
reasoning  applied  to  Fig.  7,  we  see  that  considerable  electro- 
motive stresses  would  be  set  up  by  the  revolution  of  A,  alter- 
nating in  direction  at  each  half  revolution.  In  fact  A  is  the 
armature  of  a  simple  alternating  dynamo,  having  two  poles 
N  and  S,  so  called  from  their  magnetic  relations  (see  Fig.  3). 

We  have  not  thus  far  considered  the  source  of  the  electro- 
magnetic field  involved.  It  may  be  obtained  as  shown  by 
utilizing  the  electro-magnetic  stresses  set  up  by  a  wire  convey- 


ing electrical  energy  or  on  a  small  scale  from  permanent  mag- 
nets. The  essential  fact,  however,  is  that  by  forcing  a  wire 
through  a  region  of  electro-magnetic  stress,  electromotive 
stresses  are  set  up  in  that  wire,  the  action  in  every  case  being 
in  such  direction  as  to  compel  us  to  do  work  on  the  wire. 
This  work  appears  as  electrical  energy  in  the  circuit  including 
the  moving  wire. 

Now  return  to  Fig.  5  and  consider  the  effect  if  the  wire  A  is 
carrying  a  steady  flow  of  electrical  energy.  It  will  set  up 
electro-magnetic  stresses  about  it  as  already  described.  If 
the  current  be  downward  into  the  paper  in  A  these  stresses 
will  be  opposed  to  the  stresses  in  the  field.  Inasmuch  as  we 
have  seen  that  in  setting  up  such  a  current  work  had  to  be 
done  in  forcing  the  wire  into  the  field,  it  follows  that  given 
such  a  current,  there  must  be  between  its  field  and  that  of  the 
coil  a  repulsive  force  which  had  to  be  overcome  by  doing  the 
work  aforesaid.  In  other  words,  there  must  have  been  a 
tendency  to  throw  A  out  of  the  field  of  the  coil.  Just  as 


ELEMENTARY  PRINCIPLES.  17 

work  had  to  be  spent  to  produce  electrical  energy  in  A,  so 
electrical  energy  will  be  spent  in  keeping  up  the  stresses 
around  A  that  tend  to  drive  it  out  of  the  magnetic  field.  If 
the  current  in  A  were  in  the  other  direction  the  stresses  in  its 
field  and  that  of  the  coil  would  be  concurrent  instead  of 
opposed  and  their  resultant  would  tend  to  draw  wire  and  coil 
together,  /.  ^.,  work  would  have  to  be  spent  to  keep  them 
apart.  This  is  the  broad  principle  of  the  electric  motor.  It 
is  sometimes  referred  to  as  simply  a  reversal  of  the  dynamo, 
but  it  really  makes  no  difference  whether  the  structure  in 
which  the  action  just  described  takes  place  is  well  fitted  to 
generate  current  or  not.  Given  a  magnetic  field  and  a  wire 
carrying  electrical  energy,  and  there  will  be  a  force  between 
them  depending  in  direction  on  the  directions  of  the  electro- 
magnetic stresses  belonging  to  the  two.  If  either  element  is 
arranged  so  as  to  move  and  still  keep  up  a  similar  relation 
of  these  stresses  we  have  an  electric  motor.  Whether  so 
arranged  as  to  fulfill  this  condition  with  alternating  currents, 
or  in  such  manner  as  to  require  currents  in  one  direction  only, 
the  principle  is  the  same. 

So  far  as  unidirectional  or  "continuous"  currents  are  con- 
cerned they  are  usually  obtained  from  dynamo  electric  machines 
similar  in  principle  to  Fig.  8.  This  machine,  if  the  ends  of 
the  winding  on  the  armature  be  connected  to  two  metal  rings 
insulated  from  each  other,  serves  as  a  source  of  alternating 
currents  which  can  be  taken  off  the  two  rings  by  brushes 
pressed  against  them.  If  it  is  necessary  to  obtain  currents  in 
one  direction  only,  this  can  be  readily  done  by  reversing  the 
connection  of  the  outside  circuit  to  the  windings  at  the  same 
moment  that  the  current  reverses  in  them.  The  simplest  way 
of  doing  this  is  by  a  "two  part  commutator,"  such  as  is 
shown  in  diagram  in  Fig.  9.  Here  A  is  the  shaft  surrounded 
by  an  insulating  bushing.  On  this  are  fitted  two  half  rings  C 
and  C',  of  metal  (the  commutator  segments).  On  these  bear 
brushes  B  and  B '.  If  the  ends  of  the  winding  are  connected 
to  C  and  C  and  the  brushes  are  so  placed  that  they  pass  from 
one  segment  to  the  other  at  the  moment  when  the  current  in 
the  winding  changes  its  direction,  the  direction  of  the  current 
with  respect  to  the  brushes  and  the  outside  circuit  with  which 
they  are  connected  obviously  remains  constant. 


1 8  ELECTRIC    TRANSMISSION   OF  POWER. 

In  the  actual  practice  of  dynamo  building  very  many  refine- 
ments have  to  be  introduced  to  serve  various  purposes,  but  the 
underlying  principle  remains  the  same,  /.  <?.,  to  set  up  in  a  con- 
ductor electromotive  stresses  by  dragging  it  into  and  out  of 
the  strained  region  of  ether  under  an  electro-magnetic  stress. 

According  as  the  dynamo  is  intended  for  .producing  con- 
tinuous or  alternating  currents  its  structure  is  somewhat 
modified  with  its  particular  use  in  view.  These  modifications 
extend  not  only  to  the  general  arrangement  but  to  the  details  of 
the  winding.  Alternating  dynamos  usually  have  a  more  com- 


plicated magnetic  structure  than  continuous  current  machines, 
and  are  almost  invariably  separately  excited,  /.  e.,  have  their 
magnetizing  current  supplied  from  a  generator  specialized  for 
producing  continuous  current.  The  magnetic  complication  is 
really  only  apparent,  as  it  consists  merely  of  an  increased  num- 
ber of  magnet  poles,  due  to  the  desirability  of  obtaining  toler- 
ably rapid  alternations  of  current. 

Dynamos  designed  for  producing  continuous  current  are 
modified  with  the  armature  as  a  starting  point.  The  winding 
is  very  generally  much  more  complicated  than  that  of  an  alter- 
nator and  the  commutator  that  serves  to  reverse  the  rela- 
tion of  the  windings  to  the  brushes  at  the  proper  moment  is 
correspondingly  elaborate.  The  magnetic  structure  is  usually 
comparatively  simple.  The  whole  design  is  necessarily  sub- 
ordinated to  securing  proper  commutation.  Continuous  cur- 
rent dynamos  are  almost  universally  self-excited,  that  is  the 
current  which  magnetizes  the  field  is  derived  from  the  brushes 
of  the  machine  itself.  Whatever  the  character  of  the  machine 
the  electromotive  force  generated  in  it  increases  with  the  inten- 


ELEMENTARY  PRINCIPLES.  19 

sity  of  the  magnetic  field  (that  is,  with  the  magnitude  of  the 
electro-magnetic  strains  which  affect  the  armature  conductors), 
with  the  speed  (that  is,  with  the  rate  of  change  of  electro- 
magnetic stress  about  these  moving  conductors)  and  with 
the  number  of  turns  of  wire  of  which  the  electromotive  forces 
are  added.  The  capacity  of  the  machine  for  furnishing  elec- 
trical energy  varies  directly  with  the  electromotive  force  and 
with  the  capacity  of  the  armature  conductors  for  transmitting 
the  energy  without  becoming  overheated.  Practically  all  the 
energy  lost  in  a  dynamo  appears  in  the  form  of  heat,  which 
must  be  limited  to  an  amount  which  will  not  cause  an  undue 
rise  of  temperature. 

It  is  not  the  purpose  of  this  chapter  to  deal  with  the  prac- 
tical details  of  dynamo  design  and  construction.  For  these, 
the  reader  should  consult  special  treatises  on  the  subject, 
which  consider  it  with  a  fullness  which  would  here  be  quite 
out  of  place.  Special  machines,  however,  will  be  briefly  dis- 
cussed in  their  proper  places  and  in  relation  to  the  work  they 
have  to  do. 

Having  now  considered  the  principles  which  underlie  the 
transformation  of  mechanical  into  electrical  energy,  we  may 
profitably  take  up  the  fundamental  facts  in  regard  to  the 
measurement  of  that  form  of  energy  and  the  units  in  which  it 
and  its  most  important  factors  are  reckoned. 

All  electrical  quantities  are  measured  directly  or  indirectly 
in  terms  of  the  dynamical  units  founded  upon  the  units  of 
length,  mass  and  time.  These  derived  dynamical  units  can 
serve  alike  for  the  measurement  of  all  forms  of  energy  so 
that  all  have  a  common  ground  on  which  to  stand.  As  the 
electrical  units  are  derived  directly  from  the  same  units 
that  serve  to  measure  ordinary  mechanical  effects,  electrical 
and  mechanical  energies  are  mutually  related  in  a  perfectly 
definite  way. 

A  natural  starting  point  in  the  derivation  of  a  working 
system  of  electrical  units  may  be  found  in  electro-magnetic 
stress,  such  as  is  developed  about  an  electrical  circuit  or  a 
permanent  magnet.  To  begin  with,  the  mechanical  units  that 
may  serve  to  measure  any  form  of  energy  are  derived  from  those 
of  length,  mass  and  time.  These  latter  are  almost  universally 
taken  as  the  centimetre,  gramme  and  second,  the  "  C.  G.  S.'* 


20  ELECTRIC   TRANSMISSION  OF  POWER. 

.system.  Starting  from  these  the  unit  of  force  is  that  which 
acting  for  one  second  on  a  mass  of  one  gramme  can  change 
its  velocity  by  one  centimetre  per  second.  This  unit  is  called 
the  dyne  and  as  a  magnetic  stress  it  is  equivalent  to  a  push  of 

about  of  a  pound's  weight  on  a  similar  "unit  pole" 

445,000 

one  centimetre  distant.  This  unit  is  inconveniently  small  for 
practical  use  and  before  long  some  multiple  of  it  is  likely  to 
be  given  a  special  name  and  used  for  practical  reference.  In 
fact,  one  megadyne  (i.  e.,  1,000,000  dynes)  is  very  nearly  equiva- 
lent to  the  weight  of  a  kilogramme.  Magnetic  measure- 
ments may  thus  be  made  by  direct  reference  to  the  dyne  and 
centimetre,  since  the  unit  pole  is  that  which  repels  a  similar 
pole  i  cm.  distant,  with  a  force  of  i  dyne. 

Referring  now  to  what  has  been  said  about  the  causes  which 
vary  the  electromotive  force  produced  in  a  dynamo,  we  fall  at 
once  into  the  definition  of  the  unit  electromotive  force,  which 
is  that  produced  when  field,  velocity,  and  length  of  wire  under 
induction  are  all  of  unit  value.  The  unit  electromotive  force 
is,  then,  that  which  is  generated  in  one  centimetre  of  wire 
moving  one  centimetre  per  second,  perpendicular  to  its  own 
length,  straight  across  unit  field,  which  is  that  existing  one 
centimetre  from  unit  pole  as  indicated  above.  This  unit,  too, 
is  inconveniently  small,  so  that  one  hundred  million  times  this 
quantity  is  taken  for  the  practical  unit  of  electromotive  force 
and  called  the  volt. 

The  unit  electrical  current  is  that  which  flowing  through  one 
centimetre  length  of  wire  will  create  unit  field  at  any  point 
equidistant  from  all  parts  of  the  wire  (as  when  the  wire  is 
bent  to  a  curve  of  i  cm.  radius).  One-tenth  of  this  current  is 
taken  as  the  working  unit  and  called  the  ampere. 

The  unit  electrical  resistance  (one  ohm]  is  that  through 
which  an  electromotive  force  of  one  zv//will  force  a  current  of 
one  ampere. 

The  C.  G.  S.  unit  of  work  is  that  due  to  unit  force  acting 
through  unit  distance;  that  is,  one  dyne  acting  through  one 
centimetre.  As  this  is  too  small  to  be  generally  convenient, 
ten  million  times. this  amount  is  taken  as  the  working  unit 
(called  the/02//<?).  This  is  a  little  less  than  three-quarters  of  a 
foot-pound  (exactly  .7373).  The  unit  rate  of  doing  work  is 


ELEMENTARY  PRINCIPLES.  21 

one  joule  per  second.  This  unit  rate  is  called  the  watt  and 
translating  this  into  English  measure,  one  watt  equals  T£7 
horse-power. 

Although  the  watt  is  often  spoken  of  as  an  electrical  unit,  it 
belongs  no  more  to  electrical  than  to  any  other  form  of  energy. 
It  only  remains  to  show  the  relation  of  the  watt  to  the  more 
strictly  electrical  units  just  mentioned.  Recurring  to  our 
definition  of  the  volt,  let  us  suppose  that  the  resistance  of  the 
circuit  of  which  the  moving  wire  is  a  part  is  such  that  unit 
electromotive  force  produces  unit  current  in  it.  The  stress 
between  the  field  of  the  moving  wire  and  the  other  unit  field 
in  which  it  moves  is  then  one  dyne  at  unit  distance.  In  main- 
taining this  for  one  second  at  the  given  rate  of  moving  (i  cm. 
per  second)  the  work  done  is,  as  above,  one  C.  G.  S.  unit.  At 
this  rate  if  the  E.  M.  F.  were  i  volt  and  the  current  i  ampere, 
the  work  would  be  one  joule  and  the  rate  of  doing  work  one 
watt.  If  either  E.  M.  F.  or  current  were  changed,  the  work 
would  be  proportionally  changed.  So,  the  number  of  volts 
multiplied  by  the  number  of  amperes  is  numerically  equal  to 
the  watts,  i.  e.,  we  have  obtained  the  dynamical  equivalent 
of  the  two  factors  that  make  up  electrical  energy  as  ordinarily 
reckoned.  So  the  output  of  any  dynamo  in  watts  is  deter- 
mined by  the  volt-amperes  produced,  and  we  see  the  reason 
of  the  ordinary  statement  that  746  volt-amperes  make  one  horse 
power.  This  is  always  true  whether  the  output  is  steady 
or  variable,  so  long  as  we  give  to  the  product  of  volts  and 
amperes  their  true  concurrent  values. 

What  few  other  electrical  units  appear  in  practical  work  will 
be  referred  to  in  their  proper  places. 

It  has  been  the  purpose  of  this  chapter,  not  so  much  to  set 
forth  the  ordinary  elements  of  electrical  study,  as  to  present 
these  elements  as  viewed  from  the  standpoint  of  energy.  The 
author  has  purposely  avoided  the  conception  of  electricity  as 
a  material  something,  in  favor  of  the  idea  of  stresses  in  the 
ether,  producing  strains  which  are  propagated  through  the 
ether,  thus  effecting  the  transmission  of  electrical  energy. 
Hereafter  we  shall  have  to  do  with  the  extension  of  this 
transmission  to  practical  magnitudes,  and  its  utilization  in 
the  development  of  human  industry. 


CHAPTER  II. 

GENERAL   CONDITIONS   OF    POWER    TRANSMISSION. 

THE  growth  of  human  industry  depends  on  nothing  more 
than  upon  the  possession  of  cheap  and  convenient  power. 
Labor  is  by  far  the  largest  factor  in  the  cost  of  many  manu- 
factured articles,  and  in  so  far  as  motive  power  is  cheap  and 
easy  of  application  it  tends  to  displace  the  strength  of  human 
hands  in  all  manufacturing  processes  and  so  to  reduce  the 
labor  cost  and  to  set  free  that  labor  for  other  and  less  purely 
machine-like  purposes. 

Therefore  industrial  operations  have  steadily  gravitated 
toward  regions  where  power  is  easily  procured,  often  at  the 
sacrifice  of  certain  other  advantages.  This  is  in  no  wise 
better  shown  than  by  the  growth  of  cities  around  easily  avail- 
able water  powers  even  in  regions  where  both  raw  material 
and  finished  product  became  subject  to  considerable  cost  of 
transportation.  With  the  introduction  of  the  steam  engine 
came  a  corresponding  tendency  to  gather  factories  about 
regions  of  cheap  fuel.  These  localities,  like  those  in  which 
water  power  is  plentiful,  seldom  coincide  with  centres  of  cheap- 
material  and  transportation,  so  that  it  has  generally  been 
desirable  to  strike  an  average  condition  of  maximum  economy 
by  transporting  the  necessary  power,  stored  in  the  form  of 
fuel,  to  some  advantageous  point. 

Experience  has  shown  however  that,  while  the  hauling  of 
coal  is  a  simple  and  comparatively  cheap  expedient,  fuel 
utilized  for  running  heat  engines  is  in  very  many  cases  so 
much  more  expensive  than  hydraulic  power  as  to  be  quite  out 
of  competition  in  cases  where  the  latter  can  be  transmitted 
with  a  reasonable  degree  of  economy  to  places  that  are  favor- 
able for  its  utilization.  And  in  general  it  is  found  that  there 
is  a  wide  field  for  the  transmission  of  power  obtained  from  a 
given  source  in  competition  with  power  from  some  other 
source  utilized  in  situ. 

The  sources  of  energy  on  which  we  may  draw  for  mechanical 


CONDITIONS  OF  POWER    TRANSMISSION.  23 

power  to  be  employed  on  the  spot  or  transmitted  elsewhere 
are  very  diversified,  although  few  of  them  are  to-day  utilized 
in  any  considerable  amount.  Taking  them  in  the  order  of 
their  present  importance  we  arrive  at  something  like  the  fol- 
lowing classification: 

I.   Fuel. 
II.  Water  power. 

III.  Wind. 

IV.  Solar  radiation. 

V.   Tidal  and  wave  energy. 

VI.   Internal  energy  of  the  earth. 

Of  these  only  the  first  two  play  any  important  part  in  our 
industrial  economy.  The  third  is  employed  in  a  very  small 
and  spasmodic  way,  the  fourth  and  fifth  although  enormous  in 
amount  are  almost  untouched,  while  the  last  is  not  at  present 
used  at  all,  owing  to  inherent  difficulties. 

I.  The  world's  supply  of  fuel  is  almost  too  great  for  intel- 
ligible description.  Aside  from  a  widely  distributed  and 
steadily  renewed  supply  of  wood,  the  extent  and  capacity  of 
available  coalfields  give  promise  that  for  a  very  long  time  to 
come  fuel  will  be  the  chief  source  of  energy.  Coal  is  found 
in  nearly  every  country  and  in  most  quite  plentifully,  while 
exploration  both  in  old  fields  and  in  new,  is  constantly  bringing 
to  light  fresh  supplies.  Many  computations  concerning  the 
probable  duration  of  the  coal  supply  have  been  made,  but  they 
are  generally  unreliable  owing  to  the  great  probability  that 
only  a  very  small  proportion  of  the  available  coal  is  as  yet 
known  to  mankind.  Certain  it  is  that  there  is  unlikely  to  be  a 
marked  scarcity  of  fuel  for  several  centuries  to  come,  even  at 
the  present  rate  of  increase  in  its  consumption.  Still  it  is 
altogether  probable  that  it  may  become  considerably  dearer 
than  at  present  within  perhaps  the  present  century,  owing  to 
the  increased  difficulty  of  working  the  older  mines  and  the 
comparative  inaccessibility  of  new  ones. 

Besides  coal  we  have  petroleum  and  natural  gas  in  unknown 
but  surely  very  great  quantities,  since  the  distribution  of  both 
is  far  wider  than  has  generally  been  supposed.  At  present  the 
cost  of  these  as  fuels  does  not  differ  widely  from  that  of  coal, 
but  appearances  indicate  that  they  are  likely  to  be  sooner 
exhausted. 


24  ELECTRIC    TRANSMISSION   OF  POll'EK. 

Every  improvement  that  is  made  in  the  generation  of  power 
by  steam  and  its  subsequent  distribution  helps  to  economize 
the  fuel  supply  and  stave  off  the  already  distant  day  when  fuel 
shall  be  scarce.  The  work  of  the  past  half  century  has  by  direct 
improvement  in  steam  practice  nearly  if  not  quite  doubled 
the  energy  available  per  ton  of  fuel.  Beyond  this  much  has 
been  done  along  collateral  lines.  Particularly,  explosive  vapor 
engines  have  been  developed  to  a  point  at  which  they  are  for 
small  powers  decidedly  more  economical  than  steam  engines. 
Gas  engines  of  moderate  size,  5  to  25  HP,  are  readily  ob- 
tained of  such  excellence  as  to  give  a  brake-horse-power  hour 
on  an  expenditure  of  little,  if  any,  more  than  25  cu.  ft.  of  ordi- 
nary gas,  reducing  the  cost  per  HPH  to  below  that  of  power 
from  a  steam  engine  of  similar  size.  Engines  using  an  explo- 
sive mixture  of  air  and  petroleum  vapor  are  at  least  equally 
economical,  in  fact  more  so  unless  the  comparison  be  made 
with  very  cheap  gas. 

These  explosive  engines  have  nearly  double  the  net  efficiency 
of  steam  engines  as  converters  of  thermal  energy  into  mechani- 
cal power,  and  are  capable  of  giving  under  favorable  circum- 
stances i  HPH  on  the  thermal  equivalent  of  about  i  pound  of 
coal. 

II.  Water  power  derived  from  streams  is  not  distributed  with 
the  same  lavish  impartiality  as  fuel,  but  nevertheless  exists  in 
many  regions  in  sufficient  amount  to  be  of  the  greatest  impor- 
tance in  industrial  operations.  Available  streams  exist  around 
almost  every  mountain  range  and  are  capable  of  furnishing  an 
amount  of  power  that  is  seldom  realized.  In  the  United  States 
the  total  horse  power  of  the  improved  water  power  is  approxi- 
mately 1,500,000.  New  England  is  especially  rich  in  this  re- 
spect, as  is,  too,  the  entire  region  bordering  on  the  Appalachian 
range.  The  Rocky  Mountains  are  less  favored,  the  available 
water  being  rather  small  in  amount  on  account  of  the  smaller 
rainfall  and  the  severe  cold  of  the  winters. 

The  Pacific  slope  is  rather  better  off  and  the  high  price  of  coal 
operates  to  hasten  the  development  of  every  practicable  power. 
All  over  the  country  are  scattered  small  water  powers,  and  one 
of  the  interesting  results  of  the  growth  'of  electrical  power 
transmission  has  been  to  bring  to  light  half  forgotten  falls  even 
in  familiar  streams.  Abroad,  Switzerland  is  rich  in  powers  of 


CONDITIONS   OF  POWER    TRANSMISSION.  25 

moderate  size,  as  is  the  entire  Alpine  region,  while  a  few  years 
of  experience  in  electrical  transmission  will  probably  cause  the 
discovery  or  utilization  of  many  water  powers  that  have  hardly 
been  considered,  even  in  highly  developed  countries.  Of  the 
world's  total  water  power  supply  we  know  little  more  than 
of  its  coal  supply,  but  it  is  quite  certain,  now  that  transmission 
of  power  over  very  considerable  distances  is  practicable,  that 
the  employment  of  the  one  will  every  year  lessen  the  relative 
inroads  upon  the  other.  And  this  is  in  spite  of  the  fact  that 
water  is  by  no  means  always  cheaper  than  steam  as  a  motive 
agent. 

III.  Wind  as  a  prime  mover  has  been  employed  on  a  rather 
small  scale  from  the  very  earliest  times.     Were  it  not  for  the  ex- 
treme irregularity  of  the  power  supplied  by  it  in  most  places,  the 
windmill  would  be  to-day  a  very  important  factor  in  the  problem 
of  cheap  power.      Unhappily  winds  in  the  same  place  vary  most 
erratically  from  the   merest  breeze  to  a  hurricane  sweeping 
along  at  the  rate  of  50  to  75  miles  an  hour.     As  all  strengths 
of  wind  within  very  wide  limits  must  be  utilized  by  the  same 
apparatus  running  at  all  sorts  of  speeds,  it  is  no  easy  matter  to 
employ  it  for  most  sorts  of  work.     It  seems  specially  unfitted 
for  electrical  work,  and  yet  several  small  private  plants  have 
obtained  good  results  from  windmills  used  in  connection  with 
storage  batteries. 

In  ordinary  winds  the  great  size  of  the  wheel  necessary  fora 
moderate  power  militates  against  any  very  extensive  use.  For 
example,  with  a  good  breeze  of  10  miles  per  hour  a  wheel  about 
twenty-five  feet  in  diameter  is  needed  to  produce  steadily  a  single 
effective  horse  power,  and  the  rate  of  rotation,  about  30  revolu- 
tions per  minute,  is  so  low  as  to  be  inconvenient  for  many 
purposes.  Hence  windmills  are  generally  used  for  very  small 
work  which  can  be  done  at  variable  speed,  such  as  pumping, 
grinding  and  the  like,  for  which  they  are  unexcelled  in  cheap- 
ness and  convenience.  For  large  work  we  can  hardly  count 
much  on  wind  power,  in  spite  of  ingenious  speculations  to  the 
contrary,  and  as  a  source  of  power  for  general  distribution  it 
is  out  of  the  question,  for  such  as  it  is  we  have  it  already  dis- 
tributed. It  must  rather  be  regarded  as  a  local  competitor  of 
distributed  power,  and  even  so  only  in  a  small  and  limited  field. 

IV.  Aside  from  being  in  a  general  way  the  ultimate  source 


26  ELECTRIC    TRANSMISSION  OF  POWER. 

of  nearly  all  terrestrial  energy,  the  sun  steadily  furnishes  an 
amount  of  radiant  energy  which  if  converted  into  mechanical 
power  would  more  than  supply  all  possible  human  needs.  Its 
full  value  is  the  equivalent  of  no  less  than  ten  thousand  horse 
power  per  acre  of  surface  exposed  to  the  perpendicular  rays  of 
.the  sun. 

This  prodigious  amount  is  reduced  by  perhaps  one-third 
through  atmospheric  absorption  before  it  reaches  the  sea  level, 
and  in  cloudy  weather  by  a  very  much  larger  amount.  Never- 
theless with  clear  sunlight  the  amount  of  energy  practically 
available,  after  making  all  allowances  for  increased  absorption 
when  the  sun  is  low,  and  for  the  hours  of  darkness  in  any  given 
place,  is  very  great.  If  we  suppose  the  radiant  energy  to  be 
received  on  concave  mirrors  kept  turned  toward  the  sun  and 
arranged  so  as  to  utilize  the  heat  in  the  boiler  of  a  steam  or 
vapor  engine,  the  average  result  after  making  all  allowances 
for  losses  would  be  one  mechanical  horse  power  for  each  100 
square  feet  of  mirror-aperture,  available  about  ten  hours  per 
day.  This  efficiency  has  been  substantially  realized  in  prac- 
tice, for  such  a  solar  engine  constructed  by  M.  Mouchot 
actually  gave  nearly  a  horse  power  with  a  mirror  surface  of 
about  100  square  feet. 

Such  an  engine  would  not  be  available  at  all  times  owing  to 
clouds,  but  might  very  well  prove  useful  in  some  climates  for 
irrigation  work.  During  the  dry  season  it  could  be  operated 
almost  every  day,  and  with  the  advantage  over  windmills  of 
giving  quite  steady  power  and  at  any  convenient  speed. 
Although  not  of  general  applicability,  such  apparatus  might 
be  very  valuable  in  portions  of  the  desert  country  west  of  the 
Rocky  Mountains,  in  Mexico,  western  South  America,  northern 
Africa  and  elsewhere.  So,  if  mankind  ever  is  in  dire  need  of 
power,  the  sun  stands  ready  to  furnish  it. 

V.  Of  tidal  energy  but  little  use  has  yetbeen  made.  Here  and 
there  both  here  and  abroad  are  small  tidemills,  feebly  suggest- 
ing the  enormous  store  of  tidal  power  as  yet  unutilized.  The 
intermittent  character  of  tidal  currents  and  the  small  extent  of 
the  rise  and  fall  generally  available  make  the  practical  part  of 
the  problem  somewhat  difficult.  The  easiest  way  of  harness- 
ing the  tides  is  to  let  the  rising  water  store  itself  in  artificial 
reservoirs  or  natural  ones  artificially  improved  and  then  during 


CONDITIONS   OF  POWER    TRANSMISSION.  27 

the  ebb  to  use  it  with  water-wheels.  But  usually  the  head  is  so 
small  that  for  any  considerable  power  stored  the  area  of  reser- 
voir must  be  very  large  and  the  wheels  must  be  of  great  size  in 
order  to  make  the  stored  water  do  its  work  before  the  rising 
tide  checks  further  operations.  The  average  tide  is  seldom 
more  than  10  to  12  feet  along  our  coast,  and  of  this  hardly  more 
than  half  could  be  utilized  to  give  even  a  few  hours  of  daily 
service.  At  6  feet  available  head  about  100  cubic  feet  of  water 
must  be  stored  for  each  horse-power-minute,  even  with  the 
best  modern  turbines.  Hence  for  say  1,000  HP  available  for  5 
hours  there  must  be  impounded  30,000,000  cubic  feet  of  water, 
making  a  pond  6  feet  deep  and  almost  120  acres  in  extent. 

Tidal  operations  are  therefore  likely  to  be  restricted  to  a 
few  favored  localities  where  through  natural  configuration  of 
the  ground  natural  reservoirs  can  be  found  and  where  the  rise 
of  the  tide  is  several  times  the  figure  named.  In  rare  cases  by 
the  use  of  more  than  one  reservoir  and  outlet  work  may  be 
made  nearly  or  quite  continuous.  Still  with  all  these  difficul- 
ties the  possibilities  of  tidal  power  are  enormous  in  special 
cases.  Take  for  example  the  Bay  of  Fundy  with  its  40  feet  of 
normal  tidal  rise.  If  half  this  head  can  be  used  in  practice  30 
cubic  feet  will  be  required  per  horse-power-minute,  and  a  single 
square  mile  of  reservoir  capacity  gained  by  damming  an  estu- 
ary or  cutting  into  a  favorable  location  on  shore  will  yield 
62,000  horse  power  ten  hours  per  day  in  two  five-hour  intervals. 
Generally  speaking,  economic  conditions  are  not  favorable  for 
such  an  employment  of  the  tides,  but  in  some  localities  a 
peculiarly  fortunate  contour  of  the  shore  coupled  with  high 
local  cost  of  fuel  may  render  it  easy  and  profitable  to  press  the 
tides  into  service.  The  author  has  had  occasion  to  investigate 
a  few  cases  of  this  kind  in  which  the  commercial  outlook  was 
good.  The  main  difficulties  in  utilizing  the  tides  are  two:  first, 
the  very  variable  head;  and,  second,  the  short  daily  periods  in 
which  the  outflow  can  be  advantageously  used.  Moreover, 
these  periods  shift  just  as  the  times  of  high  tide  shift,  by  a 
little  less  than  an  hour  per  day,  so  that  if  the  power  were  used 
directly  it  would  often  be  available  only  at  very  inconvenient 
times. 

To  work  the  tides  on  a  really  commercial  scale,  therefore, 
some  system  of  storing  power  is  absolutely  necessary.  And 


28  ELECTRIC    TRANSMISSION   OF  POWER. 

since  one  would  have  to  deal  with  very  large  amounts  of 
power,  much  of  the  time  the  entire  output  of  the  plant,  the 
storage  must  be  fairly  cheap  and  efficient.  For  work  on  the 
scale  contemplated  it  is  probable  that  the  storage  battery  is 
the  most  available  method.  Used  in  very  large  units  in 
a  colossal  plant,  most  of  the  serious  objections  to  the 
storage  battery  are  in  great  measure  obviated,  since  attend- 
ance and  repairs  can  be  part  of  the  duties  of  a  regular  main- 
tenance department,  inspecting,  testing  and  repairing  damaged 
cells,  casting  and  filling  new  plates,  and  keeping  the  plant  in 
first-class  working  condition  all  the  time. 

The  cost  of  battery  would  be  of  course  a  serious  matter,  but 
not  prohibitive,  and  its  efficiency  could  probably  be  kept  as 
high  as  80  per  cent.  The  best  idea  of  the  economic  side  of 
the  case  can  be  gained  by  investigating  a  hypothetical  case  of 
tidal  storage,  based,  for  convenience,  on  the  square  mile  of 
reservoir  just  mentioned.  To  simplify  the  case  we  will 
assume  use  of  the  power  locally,  so  as  not  to  complicate  the 
situation  by  the  details  of  a  long-distance  transmission.  We 
will  take  the  generators,  which  can  be  worked  at  steady  full 
load,  at  94  per  cent,  efficiency.  Then  the  efficiency  to  the  dis- 
tributing lines  would  be 

.94  x  .80  =  .756. 

At  this  rate  the  62,000  HP  available  would  give  substan- 
tially 35,000  KW;  /.  e.,  350,000  KW-hours  daily.  Storage 
capacity  would  have  to  be  provided  for  this  whole  amount 
in  a  gigantic  battery,  weighing  about  18,000  tons  and 
costing  in  the  neighborhood  of  three  million  dollars.  To 
this,  of  course,  the  cost  of  the  electrical  and  hydraulic 
machinery  must  be  added,  and  beyond  this  must  be  reckoned 
the  really  very  uncertain  cost  of  the  reservoir  and  hydraulic 
work.  In  spite  of  all  this,  an  assured  market  for  the  output 
would  lead  to  economic  success  under  conditions  quite 
possible  to  be  realized.  If  extensive  transmission  had  to 
accompany  the  enterprise  there  would  be  still  further  loss  of 
efficiency,  so  that  the  final  figure  would  not  exceed  60  per  cent., 
which  would  reduce  the  salable  power  to  about  27,900  KW.  Evi- 
dently this  would  have  to  command  a  yery  good  price,  to  carry 
the  burden  of  the  heavy  investment,  which  would  probably 
rise  to  between  $10,000,000  and  $15,000,000.  The  cost  of 


CONDITIONS   OF  POWER    TRANSMISSION.  29 

such  an  enterprise  is  so  formidable  that  it  is  practically  out  of 
the  question,  unless  it  can  reach  a  market  for  power  in  which 
a  very  high  price  is  admissible.  When  fuel  begins  to  get 
scarce  it  will  be  profitable  to  utilize  the  tides  on  a  large  scale; 
until  then,  their  use  will  be  confined  to  isolated  cases  in  w'hich 
local  causes  lead  to  high  cost  of  other  power  and  tidal  storage 
is  unusually  cheap. 

All  these  considerations  apply  with  similar  force  to  wave 
motors,  which  have  been  often  suggested,  and  now  and  then 
used,  as  sources  of  power.  The  energy  of  the  waves  is  very 
great,  as  the  havoc  wrought  by  storms  bears  witness;  but  it  is 
most  irregular  in  amount,  and  requires  very  large  apparatus 
for  its  utilization.  What  is  worse,  the  power  is  intermittent, 
so  that  to  be  of  any  natural  advantage  it  must  be  reduced  to 
a  steady  output  by  means  of  storage  of  energy  in  some  form. 
The  periodicity  of  wave  motion  is  so  low,  roughly  about  6  to- 
10  crests  per  minute,  that  flywheels  and  the  like  are  of  little  use, 
and  storage  is  practically  reduced  to  a  question  of  compressing 
air  or  pumping  water.  Even  if  some  such  wasteful  intermedi- 
ary were  not  necessary,  and  one  could  work  directly  by  means 
of  floats  or  their  equivalent,  a  float  would  have  to  have  a  dis- 
placement of  at  least  one  ton  per  horse-power,  even  if  work- 
ing in  a  pretty  heavy  sea,  and  under  ordinary  circumstances 
several  times  that  amount  of  displacement.  At  best, 
wave  motors  are  cumbersome,  and  give  small  promise  of 
economic  development  while  other  sources  of  energy  are 
available. 

VI.  Of  the  earth's  internal  heat  energy  there  is  little  to  be 
said.  It  is  quite  unused  save  as  an  occasional  source  of  hot 
water,  and  except  in  a  very  few  cases  could  not  be  employed  at 
all,  much  less  to  any  advantage.  Immense  as  is  its  aggregate 
amount,  it  is  save  at  isolated  points,  so  far  separated  from  the 
earth's  surface  as  to  be  very  difficult  to  get  at.  Hot  springs, 
very  deep  artesian  wells,  and  some  volcanic  regions  furnish  the 
only  feasible  sources  of  terrestrial  heat  energy,  so  that  the 
whole  matter  is  only  of  theoretical  interest. 

We  see  that  at  present  only  two  sources  of  energy,  viz., 
fuel  and  water  power,  are  worthy  of  serious  consideration  in 
connection  with  the  general  problem  of  the  transmission  and 
distribution  of  power.  The  other  sources  enumerated  are  either 


3°  ELECTRIC   TRANSMISSION  OF  POWER. 

very  irregular,  uncertain  in  amount,  or  so  difficult  of  utilization 
as  to  remove  them  at  once  from  the  sphere  of  practical  work. 

Granted  then  that  fuel  and  water  power  are  and  are  likely 
long  to  remain  the  dominant  sources  of  energy,  let  us  look  more 
closely  into  their  possibilities.  From  each  energy  can  be 
readily  transmitted  and  distributed  by  any  suitable  means; 
each  in  fact  can  be  transferred  bodily  to  a  distant  scene  of 
action  without  any  transformation  from  its  own  proper  form. 
In  fact  for  certain  purposes  and  under  certain  conditions  such 
is  the  very  best  method.  Fuel  for  ordinary  heating  and  water 
for  such  uses  as  hydraulic  mining  can  be  taken  as  cases  in 
point.  In  a  more  general  way  both  fuel  and  water  for  the 
development  of  mechanical  power  may  often  profitably  be 
transferred  from  place  to  place. 

The  conditions  of  economy  in  the  transmission  of  fuel  as 
such  are  comparatively  easy  to  examine  and  define.  Coal 
may  be  produced  at  the  mine  for  a  certain  quite  definite 
cost  per  ton.  It  can  be  transported  over  railroads  and 
waterways  for  an  easily  ascertainable  price.  Such  a  trans- 
mission may  be  said  to  have  a  definite  efficiency,  as  for  example 
90  per  cent,  when  the  total  transportation  charges  against  a  ton 
of  coal  amount  to  10  per  cent,  of  its  final  value.  From  this 
standpoint  it  is  quite  possible  to  transmit  power  at  this  very 
high  efficiency  even  to  the  distance  of  hundreds  of  miles.  If 
the  final  object  be  the  distribution  of  power  on  a  large  scale,  as 
from  a  great  central  station,  this  transmission  by  transportation 
of  fuel  is  often  at  once  the  most  reliable  and  the  cheapest 
method. 

Transformation  of  the  fuel  energy  at  its  source  into  some 
other  form  for  the  purpose  of  transmission  is  generally  only 
justifiable,  first,  when  by  so  doing  fuel  not  available  for  trans- 
portation at  a  high  efficiency  can  be  rendered  valuable  by  trans- 
formation of  its  energy,  or  second,  when  it  is  to  be  utilized  at 
some  distant  point  in  a  manner  which  compels  a  loss  of  efficiency 
greater  than  that  encountered  in  transmission.  As  an  example 
of  the  first  condition  fully  one-third  of  the  coal  as  ordinarily 
mined  is  unfitted,  through  its  finely  divided  condition  or  poor 
quality,  for  transportation  over  considerable  distances.  Its 
commercial  value  is  so  small  per  ton  that  it  could  not  be  car- 
ried far  without  incurring  charges  for  carriage  amounting  to 


CONDITIONS  OF  POWER    TRANSMISSION.  31 

a  large  part  of  its  value.  Hence  every  coal  mine  accumulates 
a  mountainous  culm  pile  that  is  at  present  not  only  valueless 
but  cumbers  the  ground.  This  waste  product  could  some- 
times be  very  profitably  employed  in  generating  power  which 
could  be  transmitted  at  a  relatively  very  high  efficiency  and 
sold  at  a  good  price. 

A  specimen  of  the  second  kind  may  be  found  in  the  somewhat 
rare  case  of  power  which  must  be  used  in  small  units  scattered 
over  a  considerable  territory,  so  that  they  could  be  replaced 
with  a  great  gain  in  efficiency  by  a  single  large  generating 
station.  Such  a  state  of  affairs  might  be  found  in  certain 
mining  regions  where  coal  and  iron  mines  are  interspersed. 
This  must  not  be  confounded  with  the  very  ordinary  case  of 
distributing  energy  from  a  central  station  to  various  scattered 
points,  for  we  are  here  considering  only  the  original  source  of 
the  fuel. 

When  an  extensive  distribution  of  energy  from  a  power 
station  is  contemplated,  electrical  or  similar  transmission  of 
power  to  that  station  is  generally  economical  only  on  the  condi- 
tion above  expressed,  of  using  fuel  otherwise  valueless,  since  the 
facilities  for  transportation  to  points  at  which  power  distribu- 
tion on  a  large  scale  would  be  profitable,  are  generally  good 
and  fairly  cheap.  All  this  applies  to  piping  gas  or  petroleum 
as  well  as  to  hauling  coal,  with  the  difference  that  neither  gas 
nor  petroleum  has  any  waste  corresponding  to  culm,  and  hence 
the  transportation  of  each  of  them  becomes  a  process  entirely 
comparable  with  the  transmission  of  energy  and  directly  com- 
peting therewith.  It  has  even  been  proposed  to  pipe  coal  dust 
by  pneumatic  power  for  fuel  purposes. 

Water  power  is  by  no  means  always  cheaper  than  fuel,  but  as 
a  general  rule  it  is,  and  by  such  an  amount  that  it  can  be 
transformed  into  electrical  energy  and  transmitted  to  at  least 
a  moderate  distance  without  losing  its  economic  advantage. 
It  therefore  is  usually  the  cheapest  source  from  which  to  derive 
power  for  general  distribution  on  a  large  scale. 

It  is  very  difficult  to  give  a  clear  idea  of  the  relative  cost  of 
steam  and  water  power,  for  while  the  one  can  be  predicted  for 
any  given  place  with  fair  accuracy,  the  other  is  subject  to 
immense  variations.  Once  established,  a  water  power  plant 
can  be  operated  very  cheaply,  but  the  cost  of  developing  the 


32  ELECTRIC   TRANSMISSION  OF  POWER, 

water  power  may  be  almost  anything,  and  each  case  must  be 
figured  by  itself.  It  is  easy  to  obtain  estimates  of  the  cost  of 
developing  a  given  stream  and  to  form  a  close  estimate  of 
both  the  interest  charges  to  be  incurred  and  the  additional 
expense  of  repairs  and  of  operation.  The  cost  of  steam 
power  for  the  same  conditions  can  be  accurately  estimated. 
•The  details  of  such  estimates  we  will  discuss  later.  In  general 
one  can  only  safely  say  that  the  costs  of  steam  and  water 
power  overlap,  as  it  were,  so  that  while  the  more  easily 
developed  water  powers  are  cheaper  sources  of  energy  than 
fuel  at  any  ordinary  price,  there  are  many  cases  in  which  the 
great  cost  of  development  of  difficult  water  powers  prohibits 
competition  with  steam  except  where  fuel  is  very  dear.  Much 
depends  on  the  topography  of  the  country,  the  amount  and 
reliability  of  the  available  head  of  water,  the  price  at  which 
water  rights  can  be  obtained  and  various  other  local  conditions. 
To  utilize  the  normal  minimum  power  of  a  stream  is  gener- 
ally comparatively  easy,  while  so  to  take  account  of  high  water 
as  to  obtain  nearly  the  full  continuous  working  power  of  the 
stream  often  means  great  added  expense  for  storage  capacity 
and  works  to  control  and  regulate  the  flow. 

In  addition  we  have  to  consider  two  distinct  phases  of  the 
comparative  cost — first,  the  cost  of  steam  and  water  as  prime 
movers  for  a  source  of  power  to  be  distributed,  and  second, 
the  relation  between  these  costs  and  that  of  steam  power  at 
the  points  where  the  distribution  takes  place. 

Given  a  proper  source  of  energy,  there  is  vast  variety  in  the 
character  of  the  work  of  transmission  and  distribution  that  is 
to  be  undertaken.  In  the  first  place  the  point  of  utilization 
may  be  distant  anywhere  from  a  few  hundred  feet  to  many 
miles,  and  at  that  point  the  object  may  be  the  delivery  of 
mechanical  power  in  a  single  unit,  in  one  or  several  groups  of 
allied  units,  in  one  or  several  widely  scattered  groups,  or 
finally  for  transformation  into  some  other  form  of  energy  in 
the  most  direct  way  possible. 

There  is  no  single  method  of  power  transmission  which 
meets  in  the  best  possible  manner  all  these  widely  varying  con- 
ditions. Although  electrical  transmission  is  the  most  general 
solution  of  the  difficult  problem  in  hand,  there  are  cases  in 
which  other  methods  are  preferable  and  should  be  adopted. 


CONDITIONS   Ot<    POWER    TRANSMISSION.  33 

Those  besides  electric  transmission  which  have  come  into  con- 
siderable use  are  the  following: 
I.    Wire  Rope  Transmission. 
II.    Hydraulic  Transmission. 

III.  Compressed  Air  Transmission. 

IV.  Gas  Transmission. 

It  will  be  well  to  look  into  the  distinguishing  characteristics 
of  these  and  their  relation  to  electrical  transmission  with  the 
purpose  of  finding  the  advantages  and  limitations  of  each,  so 
that  the  proper  economic  sphere  of  each  may  be  determined, 
before  taking  up  the  electrical  work  which  forms  the  main 
subject  of  this  volume.  Each  method  will  be  found  to  have 
its  own  legitimate  place. 

I.  The  transmission  of  power  by  wire  ropes  is  merely  a  very 
useful  extension  of  the  ordinary  process  of  belting.  Belts  are 
made  of  material  which  will  not  stand  exposure  to  the-weather, 
and  which  being  of  low  tensile  strength  is  heavy  and  bulky  in 
proportion  to  the  power  transmitted.  The  advantage  of  wire 
rope  over  belting  lies  in  its  high  tensile  strength  and  freedom 
from  deterioration  when  used  out  of  doors.  To  gain  the 
fullest  benefit  from  these  properties  it  is  necessary  to  use 
light  ropes  driven  at  high  speed. 

It  should  be  borne  in.  mind  that  the  power  transmitted  by 
anything  of  the  nature  of  belting  depends  directly  on  the 
speed  and  the  amount  of  pull  exercised.  If  the  force  of 
the  pull  is  100  pounds  weight  and  the  speed  of  belt  or 
rope  is  4,000  feet  per  minute  the  amount  of  power  trans- 
mitted is  400,000  foot-pounds  per  minute  or  (since  i  horse 
power  is  33,000  foot-pounds  per  minute)  about  12  HP.  The 
greater  the  speed  the  more  power  transmitted  with  the 
same  pull,  or  the  less  the  pull  for  the  same  power.  Wire  rope 
can  be  safely  run  at  a  considerably  higher  speed  than  belting 
and  is  much  stronger  in  proportion  to  its  size  and  weight.  It 
does  not  often  replace  belting  for  ordinary  work  for  the  reason 
that  owing  to  its  small  «size  it  does  not  grip  ordinary  pulleys 
anywhere  nearly  in  proportion  to  its  strength.  Hence  to  best 
take  advantage  of  its  ability  to  transmit  large  powers  the  rope 
speed  must  be  high  and  the  pulleys  unusually  large  in  diameter 
to  give  sufficient  surface  of  contact.  Such  large  wheels  are 
inconvenient  in  most  situations,  and  as  the  alternative  is  a 


34  ELECTRIC    TRANSMISSION  OF  POWER. 

number  of  ropes  which  are  troublesome  to  care  for,  rope  driv- 
ing save  for  outdoor  work  is  rather  uncommon. 

A  typical  rope  transmission  is  shown  diagrammatically  in  Fig. 
10.  Here  A  and  B  are  two  wheels,  usually  of  cast  iron,  gener- 
ally from  5  to  15  feet  in  diameter  and  with  deeply  grooved  rims. 
They  are  connected  by  a  wire  rope  perhaps  from  y?,  inch  to 
\y^  inch  in  diameter,  which  serves  to  transmit  the  power  as  the 


FIG.  10. 

wheels  revolve.  The  rope  speed  is  usually  from  3,000  to  5,000 
feet  per  minute,  sometimes  as  high  as  6,000.  The  distance 
between  the  centres  of  A  and  B  may  be  anything  required  by 
the  conditions  up  to  four  or  even  five  hundred  feet.  Greater 
distances  are  seldom  attempted  in  a  single  span,  as  if  the  rope 
is  not  to  be  overstrained  fry  its  own  weight  it  must  be  allowed 
to  sag  considerably,  compelling  the  pulleys  to  be  raised  to 
keep  it  clear  of  the  ground,  and  subjecting  it  to  danger  from 
swaying  seriously  by  reason  of  wind  pressure  or  other  acci- 
dental causes. 

The  rope  employed  is  of  special  character.     The  material  is 
the  best  charcoal  iron  or  low  steel,  and  the  strands  are  usually 


FIG.  ii. 

laid  around  a  hemp  core  to  give  added  flexibility.  The  rope 
generally  employed  in  this  country  is  of  six  strands  with  seven 
wires  per  strand,  and  is  shown  in  cross  section  in  Fig.  n. 
Even  with  the  hemp  core  there  is  still  in  an  iron  rope  sufficient 
resistance  to  bending  to  make  the  use  of  pulleys  of  large 
diameter  necessary.  Sometimes  each  separate  strand  is  made 
with  a  hemp  core  or  is  composed  of  nineteen  small  wires 


CONDITIONS  OF  POWER    TRANSMISSION.  35 

instead  of  seven  larger  ones,  to  increase  the  flexibility  and  to 
make  it  possible  to  use  smaller  sheaves  and  drums,  as  in 
hoisting  machinery. 

Steel  rope  is  slightly  more  costly  than  iron,  but  gives- 
greater  durability.  The  wheels  on  which  these  ropes  run  are 
furnished  with  a  deep  groove  around  the  circumference,  pro- 
vided with  a  relatively  soft  packing  at  the  bottom  on  which 
the  rope  rests,  and  which  serves  to  increase  the  grip  of  the 
rope  and  to  decrease  the  wear  upon  it.  Fig.  12  shows  a  section 


FIG.  12. 

of  the  rim  of  such  a  wheel.  The  bushing  at  the  bottom  of  the 
groove,  upon  which  the  rope  directly  bears,  has  been  made  of 
various  materials,  but  at  present  leather  and  specially  prepared 
rubber  are  in  most  general  use.  The  small  pieces  of  which 
the  bushing  is  composed  are  cut  to  shape  and  driven  into  the 
dovetailed  recess  at  the  bottom  of  the  groove.  The  bushings 
have  to  be  replaced  at  frequent  intervals,  and  the  cables  them- 
selves have  an  average  life  of  not  much  over  a  year. 

When  a  straightaway  transmission  of  a  few  hundred  feet  is 
necessary,  when  the  power  concerned  is  not  great  and  the  size 
of  the  pulleys  is  not  a  serious  inconvenience,  this  transmission 
by  wire  rope  is  both  very  cheap  and  enormously  efficient. 
No  other  known  method  can  compete  with  it  within  these 
somewhat  narrow  limitations.  For  a  span  of  ordinary  length 
and  the  usual  rope  speeds  the  efficiency  has  been  shown  by 
experiment  to  be  between  96  and  97  per  cent.  At  a  dis- 


36  ELECTRIC   TRANSMISSION  OF  POWER. 

tance  of  four  to  five  hundred  feet  the  weight  and  sag  of  the 
rope  becomes  a  very  serious  inconvenience  and  the  arrange- 
ment has  to  be  modified.  Perhaps  the  most  obvious  plan  is 
to  introduce  a  sheave  to  support  the  slack  of  the  cable,  as 
shown  in  Fig.  13. 

On  longer  spans  several  sheaves  become  necessary  and  both 
the  slack  and  the  tight  portions  of  the  cable  need  such  support. 


FIG.  13. 

In  cable  railway  work,  the  most  familiar  instance  of  power 
transmission  by  wire  ropes,  numerous  sheaves  have  to  be 
employed  to  keep  the  cable  in  its  working  position  in  the 
somewhat  contracted  conduit.  These  reduce  the  efficiency  of 
the  system  considerably,  so  that  the  power  taken  to  run  the 
cable  light  is  often  greater  than  the  net  power  transmitted. 
In  aerial  cable  lines  multiple  sheaves  are  seldom  used,  and  the 
more  usual  procedure  is  to  subdivide  the  transmission  into 
several  independent  spans,  thus  lessening  swaying  and  sagging 
as  well  as  the  length  of  rope  that  must  be  discarded  in  case  of  a 
serious  break.  This  device  is  shown  in  Fig.  14.  It  employs 
intermediate  pulley  stations  at  which  are  installed  double 

Q      ()      O      O 

FIG.  14. 

grooved  pulleys  to  accommodate  the  separate  cables  that 
form  the  individual  spans.  Such  a  pulley  is  shown  in 
section  in  Fig.  .15.  The  spans  may  be  three  or  four  hundred 
feet  long;  as  soon  as  the  length  gets  troublesome  another 
pulley  station  is  employed.  There  is  necessarily  a  certain 
small  loss  of  energy  at  each  such  station.  This  is  approxi- 
mately proportional  to  the  number  of  times  the  rope  passes 
over  a  pulley.  From  the  best  experimental  data  available  the 


CONDITIONS  OF  POWER    TRANSMISSION. 


37 


efficiency  of  a  rope  transmission  extended  by  separate  spans  is 
nearly  as  follows: 


Number  of  spans, 
Per  cent,  efficiency, 


12345         6789       10 
.96     .94     .93     .91     .89     .87     .86     .85     .84     .82 


These  figures  are  taken  to  the  nearest  per  cent,  and  are  for 
full  load  only.     At  half  load  the  loss  in  each  case  would  be 


FIG.  15. 

doubled.  For  instance  a  lo-span  transmission  at  half  load 
would  give  but  64  per  cent,  efficiency.  The  pulley  stations 
consist  of  the  double-grooved  wheel  before  mentioned  mounted 
on  a  substantial  and  rather  high  pedestal  or  frame-work. 


FIG.   16. 

In  this  country  a  timber  frame  is  generally  used;  abroad 
a  masonry  pier  is  more  common.  A  convenient  form  of 
frame-work  is  shown  in  Fig.  16.  An  idea  of  its  dimensions  may 
be  gained  from  the  fact  that  the  wheel  is  likely  to  be  6  to  10 
feet  in  diameter. 


35  ELECTRIC   TRANSMISSION  OF  POWER. 

It  is  interesting  to  note  that  the  efficiency  just  given  for  a  10- 
span  transmission  at  full  load  is  quite  nearly  the  same  as  would 
be  obtained  from  an  electrical  power  transmission  at  moderate 
voltage  over  the  same  distance,  assuming  a  unit  of  say  50  HP 
or  upward.  The  first  cost  of  the  latter  would  be  considerably 
higher  than  that  of  the  rope  transmission,  but  the  repairs 
would  almost  certainly  be  less  than  the  replacements  of  cable, 
bringing  the  cost  per  HP  at  full  load  to  about  the  same  fig- 
ure by  the  two  methods. 

From  actual  tests  of  electrical  apparatus  we  have  the 
following  efficiency  for  a  transmission  of  50  HP  5,000  feet,. 


FIG.  17. 

assuming  2,000  volts  and  2  per  cent,  line  loss,  which  would 
require  a  wire  less  than  one-fourth  of  an  inch  in  diameter. 
Efficiency  at  full  load  81  per  cent.,  at  half  load  72. 
Except  at  full  load  the  electrical  transmission  has  a  very 
material  advantage.  This  advantage  would  be  greatly  in- 
creased if  the  transmission  were  in  anything  but  a  straight 
line.  An  electric  line  can  be  carried  around  any  number  of 
corners  without  loss  of  efficiency,  while  a  rope  transmission 
cannot.  If  it  becomes  needful  to  change  the  direction  of  a 
rope  drive  it  is  done  at  a  station  provided  with  a  pair  of  rope 
wheels  connected  by  bevel  gears  set  at  any  required  angle. 
Fig.  17  shows  such  a  station  in  diagram.  The  loss  of  energy 
in  such  a  pair  of  bevel  gears  amounts  to  from  7  to  10  per 


CONDITIONS   OF  POWER    TRANSMISSION.  39 

cent.,  more  often  the  latter.  The  bevel  gears  may  be 
avoided  by  a  sheave  revolving  in  a  horizontal  plane  and 
carrying  the  turn  in  the  cable,  but  while  this  arrangement  is 
tolerably  efficient  it  greatly  decreases  the  life  of  the  rope. 

From  what  has  been  said  it  will  be  seen  that  while  cable 
transmission  is  for  short  distances  in  a  straight  line  both  cheap 
and  very  efficient,  at  4,000  to  5,000  feet  it  is  equalled  and  sur- 
passed in  efficiency  by  electric  transmission,  with  lesser  main- 
tenance although  greater  first  cost.  The  steel  rope  for  a  50 
HP  transmission  of  5,000  feet  would  cost  about  $400,  and 
replacement  brings  a  considerable  charge  against  each  HP 
delivered.  If  the  transmission  is  not  straight  away  or  if 
branches  have  to  be  taken  off  en  route  the  efficiency  of  the 
system  is  considerably  reduced  by  gear  stations,  while  even 
aside  from  these  the  efficiency  is  high  only  at  or  near  full  load. 
But  the  general  simplicity  and  cheapness  of  cable  transmission 
has  made  it  a  favorite  method,  and  there  have  been  many  such 
installations,  some  of  them  of  a  quite  elaborate  character. 
Most  of  them  are  small,  since  the  amount  of  power  that  can 
be  transmitted  by  a  single  rope  is  limited  to  250  or  300  HP. 
Ropes  suited  to  a  larger  power  are  too  heavy  and  inflexible; 
i^  inch  is  about  the  greatest  practicable  diameter  of  cable, 
and  even  this  requires  pulleys  between  15  and  20  feet  in 
diameter  for  its  proper  operation. 

One  of  the  oldest  and  most  considerable  rope  transmissions 
is  that  at  Schaffhausen,  where  power  derived  from  the  falls  of 
the  Rhine  is  utilized  for  distribution  to  a  number  of  factories 
in  the  vicinity.  As  a  typical  case  it  is  worth  more  than  a  pass- 
ing mention. 

The  turbine  house  contains  three  vertical  shaft  turbines 
aggregating  760  HP,  working  under  about  15  feet  head  and 
geared  to  a  common  horizontal  shaft.  Any  turbine  can  be 
put  out  of  gear  at  will.  The  main  shaft  makes  80  revolutions 
per  minute  and  carries  loose  upon  it  the  main  driving  pulleys, 
each  of  which  is  14'  9"  in  diameter.  These  are  connected  by 
bevel  gears  which  mesh  into  a  pair  carried  on  studs  driven  by 
the  shaft.  Thus  if  there  is  any  difference  in  the  tension  of 
the  two  main  cables  the  pulley  carrying  the  lesser  load  is 
accelerated  and  the  rope  tensions  are  brought  to  equality. 
From  the  turbine  house  the  main  line  crosses  to  the  left  bank 


40  ELECTRIC    TRANSMISSION  OF  POWER. 

of  the  Rhine  and  is  turned  at  right  angles  by  a  bevel  gear 
station.  Then  follow  three  spans  along  the  river  bank  and 
then  a  second  oblique  gear  station.  The  driven  span  of  this 
terminates  the  main  transmission,  of  which  the  total  length  is 
about  2,000  feet.  From  each  pulley  station  power  is  taken  off 
laterally  by  shafts  or  ropes  for  driving  various  factories,  and 
beyond  the  main  cables  are  smaller  ones  to  continue  the  dis- 
tribution. In  all  more  than  a  score  of  customers  are  served 
and  the  nominal  HP  supplied  is  over  600.  The  charge  for 
power  is  from  $24  to  $30  per  HP  per  year.  With  various 
renewals,  extensions  and  modifications  the  system  has  been 
steadily  at  work  for  more  than  a  quarter  of  a  century.  The 
principal  troubles  have  been  loss  of  efficiency  in  bad  weather 
and  a  tendency  to  oscillations  of  the  rope  and  irregularity  of 
speed  at  full  load.  One  spinning  mill  abandoned  the  trans- 
mitted power  on  account  of  the  severe  changes  of  speed. 
Financially  the  Schaffhausen  plant  has  been  a  success,  from 
the  Continental  standard  of  profit  at  least,  but  it  is  even  now 
the  only  similar  plant  where  a  general  scheme  of  distribution 
is  thoroughly  worked  out.  It  is  found  very  difficult  to  meas- 
ure equitably  the  power  delivered  to  customers. 

Let  us  now  inquire  into  the  efficiency  of  this  transmission. 
There  are  on  the  average  at  least  five  spans  of  cable  between 
the  turbines  and  the  vicinity  of  each  factory.  At  full  load 
this  would  under  the  most  favorable  circumstances  give  an 
efficiency  of  about  .90.  This  load  certainly  exists  only  in  the 
main  driving  spans.  The  power  delivered  is  in  most  cases 
transferred  to  the  working  main  shafts  by  subsidiary  rope 
drives  or  by  shafting.  If  the  average  working  efficiency  of 
this  is  above  .90  under  normal  conditions  of  load  our  experience 
with  similar  devices  elsewhere  counts  for  little.  Then  there 
are  two  bevel  gear  stations  to  be  considered,  each  of  which 
can  hardly  exceed  .92  in  efficiency.  The  total  probable  effi- 
ciency from  the  main  shaft  in  the  turbine  house  to  the  main 
shafts  of  the  factories  at  substantially  full  load  is  at  best  only 
.90  x  .90  x  -92  X  .92  =  .68,  and  at  ^  load,  which  represents 
a  much  more  likely  state  of  affairs,  about  .52.  Even  the  higher 
figure  compares  sadly  enough  with  that  just  given  for  a  single 
small  electrical  transmission  over  a  greater  distance, — .81  at 
full  load.  With  the  most  ordinary  electrical  transmission 


CONDITIONS   OF  POWER    TRANSMISSION. 


over  the  same  distance  the  net  efficiency  from  generator  shaft 
to  motor  shaft  would  be  at  least  .75  at  full  load.  It  is  worthy 
of  note  that  in  an  extension  of  the  Schaffhausen  works  made 
within  the  last  few  years,  the  cable  system  was  abandoned 
and  electrical  apparatus  installed  for  the  additional  plant. 

Although  for  a  single  straight  transmission  the  cable  system 
is  up  to  nearly  a  mile  more  efficient  than  any  other  yet  devised, 
it  is  sufficiently  evident  that  when  distribution  is  to  be  done 
involving  changes  of  direction  and  many  small  rope  drives  or 
shafts,  the  advantage  in  efficiency  very  rapidly  disappears,  and 
at  anything  but  full  load  the  system  is  very  far  from  efficient. 

Under  all  circumstances  the  need  of  replacing  the  cables 
every  year  or  so  causes  a  high  rate  of  maintenance.  The  fol- 
lowing table,  giving  the  sizes  of  iron  wire  cables  and  pulleys 
necessary  for  transmitting  various  amounts  of  power,  will  help 
to  give  a  clearer  idea  of  the  conditions  of  cable  transmission 
and  aid  in  defining  its  limited  but  useful  sphere.  Speed  is  given 
in  revolutions  per  minute  and  pulley  diameter  is  the  smallest 
permissible.  These  figures  are,  as  will  readily  be  seen,  for  rope 


Diam.  of  Rope. 

Speed. 

Diam.  of  Pulley. 

HP. 

i" 

150 

6' 

25 

1 

140 
140 

7' 
8' 

35 
45 

1 

IOO 

80 

10' 
12' 

85 

IOO 

i" 

80 

T4' 

140 

«r 

80 

14' 

150 

speeds  of  not  far  from  3,000  feet  per  minute.  This  can  fre- 
quently be  safely  raised  to  5,000  with  somewhat  larger  pulleys 
than  those  given  and  increased  revolutions,  while  for  steady 
loads  the  tension  can  be  slightly  augmented  without  danger. 
So  while  the  figures  given  are  those  suitable  for  ordinary 
running  with  a  good  margin  of  capacity,  the  HP  given  can  be 
nearly  doubled  when  all  conditions  are  favorable. 

II.  Noting  then  that  cable  transmission  does  excellent  work 
in  its  proper  place,  but  is  unsuited  for  the  distribution  of  power 
or  for  transmissions  more  than  3,000  to  4,000  feet  in  length,  we 


42  ELECTRIC   TRANSMISSION  OF  POWER. 

may  pass  to  the  hydraulic  method  of  transmitting  and  distrib- 
uting power.  This  in  its  crude  form  of  small  water  motors 
attached  to  ordinary  city  mains  is  very  familiar,  but  nothing 
more  extensive  has  been  attempted  in  this  country.  Abroad 
there  are  a  number  of  hydraulic  power  plants  specially  intended 
for  the  distribution  of  power  for  general  use,  and  the  method  is 
one  which  has  been  fairly  successful.  There  are  two  distinct 
types  of  hydraulic  plant,  one  utilizing  such  pressure  as  is 
available  naturally  or  by  pumping  to  reservoirs,  the  other 
employing  very  high  artificial  pressures,  up  to  750  pounds  per 
square  inch,  and  used  only  for  special  purposes. 

There  are  somewhat  extensive  works  of  the  former  kind  at 
Zurich,  Geneva,  and  Genoa,  the  effective  head  of  water  being 
in  each  case  not  far  from  500  feet.  In  each  case  the  power 
business  has  been  an  outgrowth  of  the  municipal  water  supply 
system.  At  Zurich  and  Geneva  elevated  reservoirs  are  supplied 
by  pumping  stations  driven  by  water  power.  At  Genoa  the 
head  is  a  natural  one,  20  miles  from  the  city,  and  much  of  the 
fall  is  utilized  18  miles  from  Genoa  in  driving  the  fine  constant 
current  electric  plant  described  elsewhere  in  this  volume. 

At  Zurich  there  is  in  addition  to  the  ordinary  low  pressure 
water  system  a  special  high  service  reservoir  supplying  power 
to  a  large  electric  station  and  to  small  consumers.  Water  is 
pumped  6,000  feet  into  this  reservoir  through  an  i8-inch  main 
and  the  total  power  service  from  both  systems  is  something 
like  500  HP,  reckoned  on  a  ten-hour  basis.  The  price  charged 
is  from  $37  to  $80  per  HP  per  year. 

The  Geneva  plant  is  on  a  much  larger  scale,  the  total  turbine 
capacity  being  about  4,500  HP.  Here,  as  at  Zurich,  there  are 
two  sets  of  mains,  one  at  nearly  200  feet  head,  the  other  at 
about  450.  Both  supply  water  for  both  power  and  general 
purposes.  The  high  pressure  service  reservoir  is  about  2^ 
miles  from  the  city  and  the  working  pressure  is  supplied 
indifferently  from  this  or  from  the  pumps  direct.  There  is  an 
electric  light  plant  with  600  HP  in  turbines  driven  by  the 
pressure  water  and  a  large  number  of  smaller  consumers. 
Water  is  supplied  to  the  electric  light  company  for  as  low  as 
$15  per  HP  per  year. 

Both  these  installations  are  extensions  of  the  city  water 
service,  and  have  done  excellent  work.  Operated  in  this  way 


CONDITIONS   OF  POWER    TRANSMISSION. 


43 


the  economic  conditions  are  somewhat  different  from  those  to 
be  found  in  a  hydraulic  plant  established  by  private  enterprise 
for  power  only.  An  inquiry  into  the  efficiency  of  such  a 
system  may  be  fairly  based  on  the  facts  given.  At  Zurich,  for 
example,  the  efficiency  from  turbine  shaft  to  reservoir  cannot 
well  exceed  .75.  The  distributing  mains  must  involve  a. loss 
of  not  less  than  10  per  cent.,  while  the  motors  cannot  be 
counted  on  for  an  efficiency  of  over  .75.  The  total  efficiency 
from  turbine  shaft  to  motor  shaft  is  then  about  .75  X  -75  X  .90 
=  50.6  per  cent.  The  character  of  the  motors  has  an  impor- 


FIG.  18. 

tant  influence  on  the  economy  of  the  system,  particularly  at 
low  loads.  The  motors  most  used  particularly  for  small  powers 
are  oscillating  water  engines  of  the  type  shown  in  Fig.  18.  The 
form  shown  is  made  by  Schmid  of  Zurich.  It  possesses  in  com- 
mon with  all  others  of  similar  construction  the  undesirable 
property  of  taking  a  uniform  amount  of  water  at  uniform  speed, 
quite  irrespective  of  load.  The  mechanical  efficiency  falls  off 
like  that  of  a  steam  engine,  friction  being  nearly  constant. 
Better  average  results  are  secured  with  impulse  turbines  (see 
Chapter  IX.)  of  which  the  efficiency  varies  but  little  as  the  load 
falls  off,  or  for  high  rotative  speeds  with  impulse  wheels  like  the 


44  ELECTRIC    TRANSMISSION   OF  POWER. 

Pelton,  shown  in  Fig  19,  as  adapted  for  motors  of  moderate 
power.  At  half  load,  /.  e.,  half  flow,  the  losses  in  distributing 
mains  would  be  reduced  to  about  one-third,  while  the  efficiency 
of  the  engine  motors  would  certainly  not  be  lowered  by  less, 
than  5  per  cent.  The  total  half  load  efficiency  would  then  be 
•75  X  .97  X  .70  —  50.9  per  cent.,  actually  a  trifle  higher  than 
at  full  load.  This  rather  remarkable  property  is  shared  by 
electric  transmissions  wherein  the  line  loss  is  fairly  large,  and 
is  occasionally  of  value. 


FIG.  19. 

The  second  type  of  hydraulic  distribution  of  power  is  that 
at  very  high  pressures  and  employing  a  purely  artificial  head. 
The  pressures  involved  are  usually  700  to  800  pounds  per  square 
inch,  and  a  small  amount  of  storage  capacity  is  gained  by 
employing  what  are  known  as  hydraulic  accumulators,  fed  by 
the  pressure  pumps.  These  accumulators  are  merely  long 
vertical  cylinders  adapted  to  withstand  the  working  pressure, 
which  is  kept  up  by  a  closely  fitting  and  enormously  heavy 
piston.  The  distribution  of  power  is  by  iron  pipes  leading  to- 
the  various  water  motors.  This  high  pressure  water  system 
is  a  device  almost  peculiar  to  England,  and  has  been  slow  in 
making  headway  elsewhere.  Its  peculiar  advantage  is  in  con- 
nection with  an  exceedingly  intermittent  load,  such  as  is 


CONDITIONS  OF  POWER    TRANSMISSION.  45 

obtained  from  cranes,  hoists,  and  the  like.  This  is  for  the 
reason  that  with  a  low  average  output  a  comparatively  small 
engine  and  pump  working  continuously  at  nearly  uniform  load 
can  keep  the  accumulators  charged,  while  the  rate  of  output 
of  the  accumulators  is  enormous  in  case  of  a  brief  demand  for 
very  great  power. 

Power  plants  on  this  hydraulic  accumulator  system  are  in 
operation  in  the  cities  of  London,  Liverpool,  Hull,  and  Bir- 
mingham, England,  and  at  Marseilles,  France.  The  Lon- 
don plant  is  the  most  important  of  those  mentioned,  con- 
sisting of  three  pumping  and  accumulator  stations  and  about 
60  miles  of  mains.  The  total  number  of  motors  operated  was 
in  1892  about  1,700.  The  charges  are  by  meter,  and  are  based 
on  intermittent  work,  being  quite  prohibitive  for  continuous 
service — from  $200  to  $500  per  effective  HP  per  year  of  3,000 
hours.  The  largest  accumulators  have  pistons  20  inches  in 
diameter  and  23  feet  stroke,  giving  a  storage  capacity  of  only 
24  horse-power-hours  each.  While  very  convenient  for  the 
supply  of  power  for  intermittent  service  only,  this  system,  like 
hydraulic  supply  at  low  pressure,  is  rather  inefficient,  the  more 
so  as  it  has  been  found  advisable  to  employ  hydraulic  motors 
of  the  piston  type,  although  special  Pelton  motors  have  been 
used  in  some  cases. 

Any  hydraulic  system  suffers  severely  from  the  inefficiency 
of  pump  and  motors  and  from  loss  of  head  in  the  pipes.  The 
amount  of  power  that  can  be  transmitted  in  the  mains  is  quite 
limited,  since  the  permissible  velocity  is  not  large.  About 
3  feet  per  second  is  customary — more  than  this  involves 
excessive  friction  and  danger  from  hydraulic  shock.  At  this 
speed  a  pipe  about  2  feet  in  diameter  is  necessary  to  trans- 
mit 500  HP  under  500  feet  head.*  The  power  delivered  in- 
creases directly  with  the  head,  but  as  the  pressure  increases 
the  largest  practicable  size  of  pipe  decreases,  and  on 
the  high  pressure  systems  nothing  larger  than  12  inches 
has  been  attempt  xl,  and  even  this  requires  the  use  of  solid 
drawn  steel. 

Whatever  the  size  of  pipe  the  loss  in  head  is  quite  nearly 
inversely  as  the  diameter  and  directly  as  the  square  of  the 
velocity.  Even  for  high  pressure  systems  this  loss  is  by  no 
means  negligible,  since  the  pipes  used  are  rather  small. 

*  Cost  per  mile  laid  in  average  unpaved  ground  about  $15,000. 


46 


ELECTRIC    TRANSMISSION   OF  POWER. 


The  following  table  gives  the  loss  of  head  in  feet  per  100  feet 
of  pipe  and  at  a  uniform  velocity  of  3  feet  per  second.  This 
applies  to  pipe  in  good  average  condition.  When  the  pipe  is 
new  and  quite  clean  the  losses  may  be  slightly  less.  If  the 
pipe  is  old  and  incrusted  the  above  losses  may  be  nearly 
doubled.  Bends  and  branches  still  further  reduce  the  working 
pressure. 


Diameter  

6" 

8" 

10" 

12* 

Loss  of  Head 

A      80 

i  62 

08 

81 

61 

Diameter  

1  6" 

iS" 

26" 

28" 

36" 

. 

16 

We  may  now  look  into  the  efficiency  of  these  high  pressure 
hydraulic  systems.  Of  the  mechanical  horse-power  applied  to 
the  pump  we  cannot  hope  reasonably  to  get  more  than  75  per 
cent,  as  energy  stored  in  the  accumulators.  Tests  on  the 
Marseilles  plant  have  shown  70  to  80  per  cent,  efficiency  between 
the  indicated  steam  power  and  the  accumulators,  the  former 
figure  at  the  speeds  corresponding  to  full  working  capacity.  As 
the  pumps  were  direct  acting  the  difference  between  brake  and 
indicated  HP  was  presumably  very  small.  The  motors  can  be 
counted  on  for  about  .75  efficiency,  and  the  losses  of  head  in 
the  pipes  for  any  ordinary  distribution  cannot  safely  be  taken 
at  less  than  5  per  cent.  Hence  the  full  load  efficiency  is  about 
•75  X  .75  X  .95  —  .53.  The  efficiency  at  full  load  is  thus  not  far 
from  that  of  the  low  pressure  system,  but  at  half  load  it  suffers 
from  the  use  of  piston  motors,  generally  necessary  on  account 
of  the  too  high  speed  of  rotary  motors  at  high  pressure. 
At  even  500  pounds  per  square  inch  pressure  the  normal  speed 
of  a  Pelton  wheel  of  say  20  HP  would  be  over  4,000  r.  p.  m.,  and 
could  not  be  greatly  reduced  without  seriously  cutting  down 
the  efficiency.  At  half  load  the  piston  motors  could  not  be 
relied  on  for  over  .65  efficiency,  reducing  the  total  efficiency, 
even  allowing  for  greatly  lessened  pipe  loss,  to  about  45  per 
cent.  On  the  whole  the  hydraulic  accumulator  system  must  be 
regarded  as  a  very  ingenious  and  occasionally  useful  freak, 
unlikely  to  find  any  permanent  use  in  power  transmission. 


CONDITIONS   OF  POWER    TRANSMISSION. 


47 


The  strongest  point  of  hydraulic  transmission  is  its  ready 
adaptability  in  connection  with  water  supply  systems  for  gen- 
eral purposes.  Skillfully  installed,  as  for  instance  at  Geneva, 
it  furnishes  convenient,  reliable,  and  fairly  cheap  motive  power. 
As  a  distinct  power  enterprise  the  high  first  cost  is  against  it, 
and  the  efficiency  is  never  really  good.  All  this  applies  with 
even  greater  force  to  the  special  high  pressure  systems,  which 
suffer  from  inability  to  cope  with  continuous  work,  thus  seri- 
ously limiting  the  possible  market.  Even  for  intermittent  ser- 
vice the  charges  are  enormously  high. 

The  methods  of  power  transmission  already  mentioned  are 
then  somewhat  limited  in  their  usefulness  by  rather  well 
defined  conditions,  which  make  their  employment  advisable  in 
some  cases  and  definitely  inadvisable  in  general. 

Transmission  by  wire  ropes  is  very  weak  in  the  matter  of 
distribution  to  numerous  small  units,  and  hydraulic  transmis- 
sion, while  escaping  this  difficulty,  is,  save  in  exceptional 
cases,  both  inefficient  and  costly. 

III.  We  may  now  pass  to  the  pneumatic  method  of  transmit- 
ting power,  which  is  far  more  general  in  its  convenient  appli- 
cability than  either  of  the  others,  and  which  is  the  only  system 
other  than  electric  which  has  been  extensively  applied  in 
practice  to  the  distribution  of  power  in  small  units,  although 
only  short  distances  have  been  involved  in  any  of  the  plants 
hitherto  operated,  and  the  possible  performance  at  long  dis- 
tances is  more  a  subject  of  speculation  than  of  reasonable  cer- 
tainty. Transmission  of  power  by  compressed  air  involves 
essentially  three  elements  :  An  air  compressor  delivering 
the  air  under  a  tension  of  from  50  to  TOO  or  more  pounds 
per  square  inch  into  a  pipe  system,  which  conveys  the  com- 
pressed air  to  the  various  motors.  These  motors  are  sub- 
stantially steam  engines  in  mechanical  arrangements,  and 
indeed  almost  any  steam  engine  can  be  readily  adapted  for  use 
with  compressed  air.  The  compressor  itself  is  not  unlike  an 
ordinary  steam  pump  in  general  arrangement.  Its  appearance 
in  the  smaller  sizes  is  well  shown  in  diagram  in  Fig.  20.  The 
system  was  originally  introduced  about  fifty  years  ago  for  mining 
purposes,  and  owed  its  early  importance  to  its  use  in  forking 
the  drills  in  the  construction  of  the  Hoosac,  Mont  Cenis  and 
St.  Gothard  tunnels.  Since  then  it  has  come  to  be  used  on  a 


48 


ELECTRIC   TRANSMISSION  OF  POWER. 


very  extensive  scale  for  drilling  operations  and  more  recently 
has  been  applied  for  the  distribution  of  power  for  general  pur- 
poses, particularly  in  Paris,  where  the  only  really  extensive 
system  of  this  kind  is  in  operation.  Its  best  field  has  been 
and  still  is  in  mining  operations  where  the  escaping  air  is  a 
welcome  addition  to  the  means  of  ventilation  and  where,  as  a 
rule,  the  distances  are  not  great. 


FIG.  20. 


Transmission  of  power  by  piping  compressed  air  has  even 
for  general  distribution  certain  very  well  marked  advan- 
tages. The  subdivision  of  the  power  can  be  carried  on 
to  almost  any  extent  and  the  motors  are  fairly  efficient, 
simple  and  relatively  cheap.  In  addition,  the  power  fur- 
nished to  consumers  can  very  easily  be  metered.  The  loss 
of  energy  can  be  kept  within  moderate  limits  and  the  mains 
themselves  are  not  liable  to  serious  breakdowns,  although 
losses  from  leakage  are  frequent  and  may  be  large.  Finally, 
the  system  is  exceptionally  safe.  On  the  other  hand,  the 
efficiency  of  the  system,  reckoned  to  the  motor  pulleys,  is 
unpleasantly  low.  The  mains  for  a  transmission  of  any  con- 


CONDITIONS   OF  POWER    TRANSMISSION.  49 

siderable  length  are  very  costly,  and  the  compressed  air  has  no 
considerable  use  aside  from  motive  power,  instead  of  being 
applicable,  like  electric  or  even  hydraulic  transmission  of 
power,  to  divers  profitable  employments  quite  apart  from  the 
furnishing  of  mechanical  energy.  To  obtain  a  clearer  idea 
of  the  nature  of  these  advantages  and  disadvantages,  let  us 
follow  the  process  of  pneumatic  transmission  from  the  com- 
pressor to  the  motor,  looking  into  each  stage  of  the  operation 
with  reference  to  its  efficiency  and  economic  value. 

The  compressor  is  the  starting  point  of  the  operation.  Fig. 
20  shows  in  section  a  typical  direct  acting  steam  compressor, 
one  of  the  best  of  its  class.  It  consists  essentially  of  the  air 
cylinder  A  and  a  steam  cylinder  B,  arranged  in  line  and  hav- 
ing a  common  piston  rod.  The  steam  end  of  the  machine 
is  simply  an  ordinary  engine  fitted  with  an  excellent  high 
speed  valve  gear  worked  by  two  eccentrics  on  the  crank 
shaft  of  the  fly  wheels  G,  which  serve  merely  to  steady  the 
action  of  the  mechanism.  The  air  cylinder  A  is  provided 
with  a  simple  piston  driven  by  an  extension  of  the  steam 
piston  rod. 

At  each  end  of  the  air  cylinder  are  automatic  poppet  valves 
E  £,  which  serve  to  admit  the  air  and  to  retain  it  during  the 
process  of  compression.  F  is  the  discharge  pipe  for  the  com- 
pressed air  leaving  the  cylinder.  In  the  compressor  shown 
there  are  two  steam  and  two  air  cylinders  connected  with  the 
cranks  90°  apart,  thus  giving  steady  rotation  in  spite  of  the 
character  of  the  work.  In  some  machines  the  pistons  and  piston 
rods  are  hollow  and  provided  with  means  for  maintaining  water 
circulation  through  them,  to  assist  in  cooling  the  air.  Round 
the  air  cylinder  is  a  water  jacket  shown  in  the  cut  just  out- 
side the  cylinder  wall.  The  purpose  of  this  is  to  keep  the 
air,  so  far  as  possible,  cool  during  compression  and  thus  to 
avoid  putting  upon  the  machine  the  work  of  compressing  air 
at  a  pressure  enhanced  by  the  heat  that  always  is  produced 
when  air  is  compressed.  And  just  here  is  the  first  weak  point 
of  the  compressed  air  system.  However  efficient  is  the 
mechanism  of  the  compressor,  all  heat  given  to  the  air  during 
compression  represents  a  loss  of  energy,  since  the  air  loses 
this  heat  energy  before  it  reaches  the  point  of  consump- 
tion. The  higher  the  final  pressure  which  is  to  be  reached  the 


50  ELECTRIC   TRANSMISSION   OF  POWER. 

more  useless  heating  of  the  air  and  the  lower  efficiency. 
Hence  the  water  jacket,  which,  by  abstracting  part  of  the  heat 
of  compression,  aids  in  averting  needless  work  on  the  air  dur- 
ing compression.  Even  the  most  thorough  jacketing  leaves 
much  to  be  desired,  generally  leaving  the  air  discharged  at 
from  200°  to  300°  F.,  more  often  the  latter.  A  cold  water 
spray  is  often  used  in  the  compressing  cylinder.  This  is  some- 
what more  thorough  than  the  jacket,  but  is  still  rather  ineffective. 
Both  serve  only  to  mitigate  the  evil,  since  they  cool  the  air  by 
absorbing  energy  from  it  and  at  best  cool  it  very  imperfectly. 
A  careful  series  of  investigations  by  Riedler,  perhaps  the  best 
authority  on  the  subject,  gives  for  the  efficiency  of  the  process 
of  compression  from  .49  10.72.  These  figures,  derived  from 
seven  compressors  of  various  sizes  and  types,  include  only 
those  losses  which  are  due  to  heat,  valve  leakage,  clearance 
and  the  like,  taking  no  account  of  frictional  losses  in  the 
mechanism.  These  are  ordinarily  about  the  same  as  in  a 
steam  engine,  say  10  per  cent.,  so  that  the  total  efficiency  of  a 
simple  compressor  may  be  taken  as  .44  to  .65,  the  latter  only  in 
large  machines  under  very  favorable  conditions.  The  most 
considerable  recent  improvement  in  compressors  is  the  division 
of  the  compression  into  two  or  more  stages,  as  the  expansion 
is  divided  in  compound  and  triple  expansion  engines.  This 
limits  the  range  of  heating  that  can  take  place  in  any  given 
cylinder  and  greatly  facilitates  effective  cooling  of  the  air. 
Riedler  has  obtained  from  two-stage  machines  of  his  own 
design  a  compressor  efficiency  of  nearly  .9.  Allowing  for  the 
somewhat  greater  friction  in  the  mechanism  the  total  efficiency 
was  found  to  be  about  .76.  In  general  then  we  may  take 
the  total  efficiency  of  the  single  stage  compressors  usually 
employed  in  this  country  as  .5  to  .6,  very  rarely  higher,  while 
the  best  two-stage  compressors  may  give  an  efficiency  slightly 
in  excess  of  .75.  For  steady  working,  .75  would  be  an  excel- 
lent result. 

We  may  next  look  into  the  action  of  the  compressed  air  in 
the  mains.  As  in  the  case  of  water,  the  frictional  resistance 
and  consequent  loss  of  pressure  varies  directly  with  the  square 
of  the  velocity  of  the  air  and  inversely  with  the  diameter  of 
the  pipe.  By  reducing  the  one  and  increasing  the  other  the 
efficiency  of  the  line  may  be  increased  at  the  cost  of  a  con- 


CONDITIONS   OF  POWER    TRANSMISSION. 


siderable  increase  in  original  outlay.  Any  attempt  to  force  the 
output  of  the  main  rapidly  increases  the  losses.  At  a  working 
gauge  pressure  of  60  pounds  per  square  inch,  which  is  in  very 
frequent  use,  the  per  cent,  of  pressure  lost  per  1,000  feet  of 
pipe  of  various  diameters  is  given  in  the  following  table — the 
velocity  being  taken  at  30  feet  per  second: 


Diameter  

i" 

2" 

3  " 

4 

5" 

12" 

24" 

36" 

48" 

Percent,  loss  

21.8 

10.9 

7.8 

5-45 

4-37 

3.66 

I.O 

0.66 

0-5 

•33 

•25 

The  friction  in  the  pipes  is  proportionally  greater  in  small 
pipes  than  in  large,  and  this  table  is  taken  as  correct  for  the 
medium  sizes.  No  allowance  is  made  for  increase  in  velocity 
through  a  long  main,  for  leakage,  nor  for  draining  traps, 
elbows,  curves  and  other  extra  resistances,  so  that  as  in  prac- 
tice the  larger  and  longer  mains  suffer  the  more  from  these 
various  causes,  the  table  will  not  be  found  widely  in  error  for 
ordinary  cases.  Very  large  straight  away  mains  will  give 
somewhat  better  results,  and  the  five  last  columns  of  the  table 
are  computed  from  Riedler's  experiments  on  the  Paris  air 
mains,  n^  inches  in  diameter  and  10  miles  long.  All  losses 
are  included.  Losses  in  the  air  mains  can  therefore  be  kept 
within  a  reasonable  amount  in  most  cases.  With  large  pipes 
and  low  velocities  power  can  be  transmitted  with  no  more  loss 
than  is  customary  in  the  conductors  of  an  electrical  system. 
Small  distributing  pipes,  however,  entail  a  serious  loss  if  they 
are  of  any  considerable  length. 

The  motor  is  the  last  element  of  pneumatic  transmission  to 
be  consideied.  Generally  it  is  almost  identical  with  an  ordi- 
nary steam  engine;  in  fact  steam  engines  have  been  often 
utilized  for  air,  and  common  rock  drills  may  be  used  indif- 
ferently for  steam  or  air  with  sometimes  slight  changes  in  the 
packing  of  the  pistons  and  piston  rods.  Some  special  air 
motors  are  in  use  with  slight  modifications  from  the  usual 
steam  engine  type.  In  most  of  these  the  air  is  used  expan- 
sively and  at  a  fairly  good  efficiency.  Tests  by  Riedler  on  the 
Paris  system  show  for  the  smaller  air  motors  an  efficiency  of  as 
high  as  85  per  cent,  so  far  as  the  utilization  of  the  available 
energy  in  the  air  is  concerned,  or  taking  into  account  the 


52  ELECTRIC   TRANSMISSION  OF  POWER. 

mechanical  losses,  70  to  75  per  cent.  Occasional  results  as 
low  as  50  to  60  per  cent,  were  obtained  even  when  the  air  was 
used  expansively,  while  if  used  non-expansively  the  total  effi- 
ciency was  uniformly  below  40  per  cent.  Tests  on  an  adapted 
steam  engine  with  Corliss  valve  gear  gave  a  pneumatic  efficiency 
of  .90,  with  a  total  efficiency  of  .81.  These  figures  are  under 
more  than  usually  favorable  conditions. 

One  of  the  principal  difficulties  with  air  motors  is  freezing 
due  to  the  sudden  expansion  of  the  compressed  air,  and  the 
congelation  of  any  moisture  carried  with  it.  It  is  quite  use- 
ful therefore  to  supply  to  the  motor  artificially  a  certain 
amount  of  heat,  sufficient  to  keep  the  exhaust  at  the  ordinary 
temperature,  especially  if  the  air  has  been  cooled  by  spray 
during  compression.  This  heating  process  is  very  frequently 
extended  so  as  not  only  to  obviate  all  danger  of  freezing  but  to 
add  to  the  output  of  the  air  motor  by  giving  to  the  compressed 
air  a  very  considerable  amount  of  energy.  The  air  is  passed 
through  a  simple  reheating  furnace  and  delivered  to  the  motor 
at  a  temperature  of  about  300°  Fahrenheit.  The  energy  de- 
livered by  the  motor  is  composed  of  that  actually  transmitted 
through  the  mains  plus  that  locally  furnished  by  the  reheater. 

The  amount  of  fuel  used  is  not  great,  usually  from  -J  to  J-  of 
a  pound  of  coal  per  horse-power-hour,  and  the  increase  of 
power  obtained  is  about  25  per  cent,  of  that  which  would 
otherwise  be  obtained  from  the  motor.  This  means  that  the 
heat  is  very  effectively  utilized.  Reheating  is  not  a  method  of 
increasing  the  efficiency  of  the  system,  as  is  sometimes  supposed, 
but  a  convenient  way  of  working  a  hot  air  engine  in  conjunc- 
tion with  an  initial  pressure  obtained  from  air  mains.  It 
increases  the  operating  expense  by  a  very  perceptible  though 
rather  small  amount  and  gains  a  good  return  in  power.  In  so 
far  it  is  desirable,  but  it  no  more  increases  the  efficiency  of  the 
pneumatic  transmission  than  would  power  from  any  other 
source  added  to  the  power  actually  transmitted. 

We  are  now  in  a  position  to  form  a  clear  idea  of  the  real 
efficiency  of  transmission  of  power  by  compressed  air.  Taking 
the  compressor  and  motor  efficiencies  already  given  and  assum- 
ing 10  per  cent,  loss  of  energy  in  the  mains,  we  have  for  the 
total  efficiency  from  indicated  horse-power  at  the  compressor 
to  brake-horse-power  at  the  motor  :  .75  x  .90  X  .80  =  .54 


CONDITIONS   OF  POWER    TRANSMISSION.  53 

for  large  two-stage  compressors  and  large  motors;  while  with 
ordinary  apparatus  it  would  be  about. 70  x  -9°  X  -75  =  -47- 
At  half  load  these  figures  would  be  reduced  to  about  .45  and 
.35  respectively.  In  operating  drills,  which  are  motors  in 
which  the  air  is  used  non-expansively  and  to  which  the  air  is 
carried  considerable  distances  through  small  pipes,  the  total 
efficiency  is  almost  always  below  rather  than  above  .30.  The 
efficiency  of  .54  given  above  cannot  well  be  realized  without 
recourse  to  artificial  heating  to  enable  the  air  to  be  used 
expansively  without  trouble  from  freezing. 

Compressed  air  has  been  mainly  used  for  mining  operations, 
where  its  entire  safety  and  its  ventilating  effect  are  strong  points 
in  its  favor.  More  rarely  it  is  employed  for  general  power 
purposes.  Of  such  use  the  Popp  compressed  air  system  in 
Paris  is  by  far  the  best  example. 

This  great  work  started  from  a  system  of  regulating  clocks 
by  compressed  air  established  a  quarter-century  ago.  Nearly 
a  decade  later  the  use  of  the  compressed  air  for  motors 
began,  and  after  several  extensions  of  the  old  plant  the  pres- 
ent station  was  built.  It  contains  four  2,000  HP  compound 
compressors,  of  which  three  are  regularly  used  and  the  fourth 
held  in  reserve.  The  steam  cylinders  are  triple  expansion, 
worked  with  a  steam  pressure  of  180  pounds.  The  air  pres- 
sure is  7  atmospheres  and  the  new  mains  are  20  inches  in 
diameter,  of  wrought  iron.  There  are  in  all  more  than  30 
miles  of  distributing  main,  most  of  it  of  12  inches  and  under  in 
diameter.  A  very  large  number  of  motors  of  sizes  from  a 
fan  motor  to  more  than  100  HP  are  in  use.  Their  total 
amount  runs  up  to  several  thousand  HP,  even  thougb  the 
majority  of  them  are  less  than  a  single  horse-power.  Except 
in  very  small  motors  reheaters  are  used,  raising  the  temperature 
of  the  air  generally  to  between  200°  and  300°  F.  The  effi- 
ciency of  the  whole  system  from  Professor  Kennedy's  investi- 
gations is  about  50  per  cent,  under  very  favorable  conditions. 
The  prices  charged  for  power  have  not  been  generally  known 
but  are  understood  to  be  somewhat  in  excess  of  $100  per  horse- 
power per  working  year.  An  interesting  addition  to  the  ap- 
paratus of  pneumatic  transmission  has  recently  appeared.  It 
is  a  modification  of  the  ancient  "trompe,"  or  water  blast,  used 
for  centuries  to  feed  the  forges  of  Catalonia,  very  simple 


54 


ELECTRIC   TRANSMISSION  OF  POWER. 


FIG.  21. 


CONDITIONS  OF  POWER    TRANSMISSION.  55 

in  operation  and  cheap  to  build.  In  its  present  improved 
form  it  is  known  as  the  Taylor  Hydraulic  Air  Compressor,  and 
an  initial  plant  of  very  respectable  size  has  been  in  highly  suc- 
cessful operation  for  nearly  two  years  past  at  Magog,  P.  Q.r 
from  which  the  data  here  given  have  been  obtained. 

The  compressing  apparatus  which  is  shown  in  Fig.  21  is  in 
principle  an  inverted'siphon  having  near  its  upper  end  a  series 
of  intake  tubes  for  air,  and  at  the  bend  a  chamber  to  collect 
the  air  which,  entrained  in  the  form  of  fine  bubbles,  is  carried 
down  with  the  water  column,  which  flowing  up  the  short  arm 
of  the  siphon  escapes  into  the  tail  race.     In  Fig.  21,  A  is  the 
penstock   delivering  water   to    the    supply   tank   B.     In   this 
tank  is  the  mouth   of  the  down  tube   C,    contracted  by  the 
inverted  cone  C  so  as  to  lower   the  hydraulic  pressure  and 
allow    ready  access  of  air   from   the    surrounding  apertures. 
The  air  bubbles  trapped  in  the  water  sweep  down  C,  which 
expands  at  the  lower  end  and  finally  enters  the  air  tank  D. 
Here  the  water  column  encounters  the  cone  K,  which  flattens 
into  a  plate  at  the  base.     Thus  spread  out  and  escaping  from 
the  air  chamber  by  the  circuitous  route  shown  by  the  arrows, 
the  air  bubbles  from  the  water  accumulate  in  the  top  of  the 
air  tank,  while  the  water  itself  rises  up  the  shaft  £,  and  flows 
into  the    tail   race  F.      The   air  in  D  is  evidently  under  a 
pressure  due  to  the  height  of  the  water  column  up  to  F,  and 
quite   independent  of  the  fall   itself,  which  consequently  may 
vary  preatly  without  affecting  the  pressure  of  the  stored  air,  a 
very   valuable   property   in    some   cases,  as  in  utilizing  tidal 
falls.     From  D  the  compressed  air  is  led  up  through  a  pipe. 
P,  for  distribution  to  the  motors.     To  get  more  pressure  it  is 
only  necessary  to  burrow  deeper  with  the  air  tank,  not  a  diffi- 
cult task  where  easy  digging  can  be  found.     The  fall  and  rate 
of   flow  determine  the  rate  at  which  the  air  is  compressed, 
and  contrary  to  what  might  be  supposed  the  process  of  com- 
pression is  quite  efficient.     It  is  quite  sensitive  to  variations 
in  the  amount  of  flow,  the  efficiency  changing  rapidly  with  the 
conditions  of  inlet,  and  since  there  certainly  is  a  limit  to  the 
amount  of  air  that  can  be  entrained  in  a  given   volume  of 
water,  the  process  is  likely  to  work  most  efficiently  at  moderate 
heads  and  with  large  volumes  of  water.     In  the  Magog  com- 
pressor about  4  cubic  feet  of  water  are   required  to   entrain 


56  ELECTRIC    TRANSMISSION   OF  POWER. 

i  cubic  foot  of  air  at  atmospheric  pressure,  and  it  is  open  to 
question  as  to  how  far  this  ratio  could  be  improved.  This 
ratio,  too,  would  be  changed  for  the  worse  rapidly  in  attempt- 
ing high  compression,  so  that  the  Magog  results  probably 
represent,  save  for  details,  very  good  working  conditions. 
The  dimensions  of  the  Magog  apparatus  are  given  in  the 
accompanying  table,  which  is  followed  by  the  details  of  one  of 
the  tests  made  by  a  very  competent  body  of  engineers. 
The  general  dimensions  of  the  compressor  plant  are: 

Supply  penstock 60  inches  diameter 

Supply  tank  at  top 8  feet  diam.  by  10  feet  high 

Air  inlets  (feeding  numerous  small  tubes) 34  2-inch  pipes 

Down  tube 44  inches  diameter 

Down  tube  at  lower  end 60  inches  diameter 

Length  of  taper  in  down  tube,  changing  from  44-inch 

to  6o-inch  diameter 20  feet 

Air  chamber  in  lower  end  of  shaft 16  feet  diameter 

Total  depth  of  shaft  below  normal  level  of  head  water about  150  feet 

Normal  head  and  fall about  22  feet 

Air  discharge  pipe 7  inches  diameter 

Flow  of  water,  cubic  feet,  minute 4292. 

Head  and  fall  in  feet !9-5O9 

Gross  water  HP 158.1 

Cubic  feet  compressed  air  per  minute,  reduced  to  atmospheric 

pressure , 1 148. 

Pressure  of  compressed  air,  Ibs 53.3 

Pressure  of  atmosphere,  Ibs 14.41 

Effective  work  done  in  compressing  air,  HP 111.7 

Efficiency  of  the  compressor,  per  cent 70.7 

Temperature  of  external  air,  Fahr 65 .2 

Temperature  of  water  and  compressed  air,  Fahr 66.5 

Moisture  in  air  entering  compressor,  per  cent,  of  saturation 68. 

Moisture  in  air  after  compression,  per  cent,  of  saturation 35. 

The  efficiency  given  is  certainly  most  satisfactory,  being 
quite  as  high  as  could  be  attained  by  a  compound  compressor 
of  tshe  best  construction  driven  by  a  turbine,  and  for  the  head 
in  question  at  a  very  much  lower  cost.  It  is  probable  that 
the  test  given  does  not  represent  the  best  that  can  be  done  by 
this  method,  and  the  indications  are  that  within  a  certain, 
probably  somewhat  limited,  range  of  heads  the  hydraulic  com- 
pressor will  give  as  compressed  air  a  larger  proportion  of  the 
energy  of  the  water  than  any  other  known  apparatus.  Just 


CONDITIONS   OF  POWER    TRANSMISSION.  57 

what  its  limitations  are  remains  to  be  discovered,  but  several 
plants  are  now  under  construction  which  will  throw  consider- 
able light  upon  the  subject. 

In  certain  cases  the  power  of  getting  compressed  air  direct 
from  hydraulic  power  by  means  of  a  simple  and,  under  favor- 
able conditions,  cheap  form  of  apparatus,  is  very  valuable, 
and  while  it  is  unlikely  to  change  radically  the  status  of  pneu- 
matic transmission,  it  is  an  important  addition  to  available 
engineering  methods.  For  dealing  with  moderate,  and  par- 
ticularly with  very  variable,  heads,  such  compression  may 
prove  to  be  an  important  intermediary. 

As  in  most  pneumatic  plants,  the  Magog  installation  is 
worked  in  connection  with  reheaters. 

IV.  In  point  of  convenience  and  efficiency  compressed  air  is 
nearer  to  electricity  for  the  distribution  of  power  over  large 
areas  than  any  other  method.  The  only  other  system  that 
approaches  them  is  the  transmission  of  gaseous  fuel  for  use 
in  explosive  gas  engines.  At  equal  pressures  one  can  send 
through  a  given  pipe  twenty  times  as  much  energy  stored  in 
gas  as  in  air.  A  good  air  motor  requires  about  450  cubic  feet 
of  air  at  atmospheric  pressure  per  indicated  HP  hour,  while  a 
gas  engine  will  give  the  same  power  on  a  little  over  20  cubic 
feet  of  gas.  But  the  cases  wherein  the  distribution  of  gas 
would  be  desirable  in  connection  with  a  transmission  over 
a  long  line  of  pipe  are  comparatively  few.  Particularly  this 
system  has  no  place  in  the  development  of  water  powers,  the 
most  important  economic  function  of  electrical  transmission. 
Nevertheless  it  must  be  admitted  that  for  simple  distribution 
of  power  a  well  designed  fuel  gas  system  is  a  formidable  com- 
petitor of  any  other  method  yet  devised,  particularly  in  the 
moderate  powers — say  from  5  to  25  HP. 

We  are  now  in  a  position  to  review  the  divers  sorts  of  power 
transmission  that  have  been  discussed,  and  to  compare  them 
with  power  transmission  by  electricity. 

Without  going  deeply  into  details,  which  will  be  taken  up  in 
due  course,  we  may  say  that  electrical  machinery  possesses  one 
advantage  to  an  unique  extent — high  efficiency  at  moderate 
loads.  Machinery  in  which  the  principal  losses  are  frictional 
is  subject  to  these  in  amount  nearly  independent  of  the  load; 
hence  the  efficiency  drops  rapidly  at  low  loads.  In  dynamos, 


5»  ELECTRIC    TRANSMISSION   OF  POWER. 

motors,  and  transformers,  however,  the  principal  losses 
decrease  rapidly  with  the  load,  so  that  within  a  wide  range  of 
load  the  efficiency  is  fairly  uniform.  Fig.  22  gives  the  efficiency 
curves  for  a  modern  dynamo,  motor,  and  transformer.  The 
generator  curve  is  from  a  large  5oo-volt  direct-current 
machine,  the  motor  curve  from  a  smaller  machine  of  the  same 
type,  and  the  transformer  curve  from  a  standard  type  of  about 
30  kilowatts  capacity.  In  the  generator  curve  the  variation  of 
efficiency  from  half  load  to  full  load  is  less  than  2  per  cent, 
in  the  motor  only  2^  per  cent.,  and  in  the  transformer 
just  ij^  per  cent.  In  addition,  the  efficiency  of  all  three 
at  full  load  is  very  high.  Hence,  not  only  is  an  electrical 
power  transmission  of  great  efficiency  if  the  loss  in  the  line  be 
moderate,  but  this  efficiency  persists  for  a  wide  range  of  load. 
As  in  hydraulic  and  pneumatic  transmission,  the  efficiency 
of  the  line  depends  on  ks  dimensions;  so  that  by  increasing 


70 


the  weight  of  copper  in  the  line,  the  loss  of  energy  may  be 
decreased  indefinitely.  And  since  the  loss  of  energy  in 
the  line  diminishes  as  the  square  of  the  current,  the  per- 


CONDITIONS  OF  POWER    TRANSMISSION.  59 

•centage  of  loss  at  constant  voltage  diminishes  directly  with 
the  load. 

Hence  the  total  efficiency  may  be  constant  or  even  increase 
from  half  load  to  full  I'oad,  even  with  a  quite  moderate  loss  in 
the  line.  In  pneumatic  and  hydraulic  transmission  this  con- 
dition may  occur,  but  only  with  large  loss  in  the  mains,  since  the 
efficiency  of  the  generator  and  motor  parts  of  these  systems 
decreases  too  rapidly  to  be  compensated  by  the  gain  in  the 
main,  unless  its  efficiency  is  low  at  full  load.  Hence,  for 
ordinary  cases  of  distribution  in  which  the  average  load  is  con- 
siderably less  than  full  load,  often  only  ^  to  ^  of  full  load, 
•electric  transmission  has  a  very  material  advantage  over  all 
other  methods.  To  appreciate  this  we  need  only  to  run  over 
the  details  of  electrical  power  transmission  and  compare  the 
results  with  those  which  we  have  obtained  for  the  other 
methods  described. 

There  are  to  be  considered  in  electrical  power  transmis- 
sion, as  in  transmission  of  every  sort,  two  somewhat  distinct 
problems: 

First,  the  transmission  of  energy  over  a  considerable  dis- 
tance and  its  utilization  in  one  or  a  few  large  units. 

Second,  the"distribution  of  power  to  a  large  number  of  small 
units  at  moderate  distances  from  the  centre  of  distribution. 
This  latter  case  may  sometimes  also  involve  the  transmission 
of  power  to  a  real  or  fictitious  centre  of  distribution.  This 
second  problem  is  the  commoner  and,  while  not  so  sensational 
as  the  transmission  of  power  at  high  voltage  over  distances  of 
many  miles,  is  of  no  less  commercial  importance. 

We  have  all  along  been  considering,  in  treating  of  transmis- 
sion of  power  by  ropes  and  by  hydraulic  and  pneumatic  engines, 
the  case  first  mentioned,  excepting  in  so  far  as  some  special 
distributions  have  been  referred  to.  We  have  already  the  data 
for  figuring  the  efficiency  of  an  electric  power  transmission 
with  large  units.  In  cases  of  this  kind  the  distance  between 
the  generator  and  motor  is  likely  to  be  much  greater  than  in 
the  case  of  distribution  to  small  motors  from  some  central 
point,  and  the  loss  in  the  line,  the  only  uncertain  figure  in  the 
transmission,  would  generally  range  from  5  to  10  per  cent. 
In  case  of  distributing  plants  intended  to  furnish  from  a  single 
point  small  units  of  power  over  a  moderate  distance,  it  is 


60  ELECl'RIC    TRANSMISSION   OF  POWER. 

generally  found  that  losses  in  the  line  of  from  2  to  5  per 
cent,  do  not  involve  excessive  cost  of  copper.  In  cases  where 
a  distribution  is  coupled  with  the  transmission  of  power  to  the 
central  point,  the  loss  from  the  distant  generator  to  the  motors 
is  in  most  cases  from  10  to  15  per  cent. 

Taking  up  first  the  transmission  of  power  from  one  or  more 
large  generators  to  one  or  more  large  motors,  we  may  take 
safely  the  commercial  efficiency  of  the  generator  as  that  given 
by  the  curve,  Fig.  22,  and  that  of  the  motors  as  at  least  as 
good  as  that  given  for  a  motor  in  the  same  figure.  The 
efficiency  of  the  line  for  moderate  distances  may  be  taken  as 
95  per  cent.  It  should  be  noted  that  the  efficiencies  of  large 
alternating  generators  and  motors  do  not  differ  materially 
from  those  shown;  in  fact,  are  quite  as  likely  to  be  above  them 
as  below  them.  We  thus  have  for  the  efficiency  in  a  transmis- 
sion of  this  kind:  94  X  95  X  93  =  84  per  cent.  This  is  largely 
in  excess  of  that  which  could  be  obtained  at  distances  of  say  a 
couple  of  miles  by  any  other  method  of  transmission. 

Even  more  extraordinary  is  the  efficiency  at  half  load  in 
this  case,  which  is  92  x  97«5  X  91  =  81.6  per  cent.  It 
should  be  borne  in  mind  that  these  efficiencies  are  taken 
from  experiments  with  standard  machines,  and  the  efficiencies 
are  those  which  can  be  realized  in  practice.  These  results 
show  the  great  advantage  to  be  derived  from  electrical  trans- 
mission when,  as  in  most  practical  cases,  full  load  is  seldom 
reached.  It  is  most  important  for  economical  operation  to 
employ  a  system  which  will  give  high  efficiency  at  low  loads, 
and  it  would  be  worth  while  so  to  do  even  if  the  efficiency  at 
full  load  were  not  particularly  good.  With  electrical  machin- 
ery, however,  there  is  no  such  disadvantage.  Even  at  one- 
fourth  load  the  efficiency  of  the  electrical  system  still  remains 
good.  It  is  nearly  73  per  cent,  on  the  assumed  data.  The 
efficiencies  thus  given  are  from  the  shaft  of  the  generator  to 
the  pulley  of  the  motor  inclusive. 

In  the  case  of  distributed  motors  supplied  from  a  central 
point  not  very  distant  from  any  of  them,  the  efficiencies  of 
generator  and  line  remain  about  as  before,  but  the  motor 
efficiencies  for  the  sizes  most  often  employed  are  below  that 
just  given.  The  average  motor  efficiency  is  largely  dependent 
on  the  skill  with  which  the  units  are  distributed.  It  has  often. 


CONDITIONS   OF  POWER    TRANSMISSION. 


61 


been  proposed  to  drive  separate  machines  by  individual  motors, 
while  in  other  cases  comparatively  long  lines  of  shafting  are 
employed,  grouping  many  machines  into  a  dynamical  unit 
operated  by  a  motor.  To  secure  economy  it  is  desirable 
on  the  one  hand  to  use  fairly  large  motors  well  loaded,  on 
the  other  hand  the  losses  in  shafting  and  belting  must  be  kept 
down. 

The  larger  the  motors,  the  better  their  efficiency  at  all 
loads  and  the  less  the  average  cost  per  HP,  but  with  small 
motors  the  cost  and  inefficiency  of  shafts  and  belts  may  be 
in  large  measure  avoided.  The  most  economical  arrange- 
ment depends  entirely  upon  the  nature  of  the  load.  Much 
may  be  said  in  favor  of  individual  motors  for  each  machine, 
but  so  far  as  total  economy  is  concerned,  this  practice  is  best 
limited  to  a  few  cases — machines  demanding  several  HP  (say 
5  or  more)  to  operate  them,  machines  so  situated  as  to  neces- 
sitate much  loss  in  transmitting  power  to  them,  and  certain 
classes  of  portable  machines.  In  applying  electric  power 
to  workshops  already  in  operation  the  group  system  will 
usually  give  the  best  results,  individual  motors  being  used 
only  for  such  machines  as  might  otherwise  cause  serious  loss 
of  power.  The  following  table  gives  the  average  full  load 
efficiencies  that  may  safely  be  expected  from  motors  of  various 
sizes,  irrespective  of  the  particular  type  employed. 


HP  of  motor  

! 

3 

5 

7% 

10 

15 

20 

25 

88 

40 

So 

75 

Per  cent,  efficiency 

72 

78 

81 

83 

85 

86 

87 

90 

00 

Qi 

These  are  commercial  efficiencies  reckoned  from  the  electri- 
cal input  to  the  mechanical  output  at  the  pulleys.  Below  5  HP 
the  efficiencies  fall  off  rapidly.  At  partial  loads  the  efficiencies 
are  somewhat  uncertain,  inasmuch  as  some  motors  are  designed 
so  as  to  give  their  maximum  efficiency  at  some  point  below 
full  load,  while  others  work  with  greater  and  greater  efficiency 
as  the  load  increases  until  heating  or  sparking  limits  the  out- 
put. The  former  sort  are  most  desirable  for  ordinary  work- 
shop use,  while  the  latter  are  well  suited  to  intermittent  work 
at  very  heavy  loads,  as  in  hoisting.  The  difference  in  the  two 
types  of  machine  is  very  material.  It  is  easily  possible  to 
procure  motors  that  will  not  vary  more  than  5  per  cent,  in 


62  ELECTRIC    TRANSMISSION  OF  POWER. 

efficiency  from  full  load  to  half  load,  and  this  even  in  machines 
as  small  as  2  or  3  HP.  We  may  now  calculate  the  efficiency 
of  an  electric  distribution  with  motors  of  moderate  size — such 
a  case  as  might  come  from  the  electrical  equipment  of  large 
factories.  The  generator  efficiency  may  be  taken  as  before 
at  .94  and  that  of  the  line  at  .95,  while  the  motors  must  be 
taken  close  account  of  in  order  to  estimate  their  collective 
efficiency.  Assuming  the  sizes  of  motors  in  close  accordance 
with  those  in  several  existing  installations  of  similar  character, 
we  may  sum  them  up  about  as  follows: 

5 3HP 

5 5  HP 

10 10  HP 

10 20  HP 

5 25  HP 

2 50  HP 

In  all  37  motors  aggregating  565  HP.  The  mean  full  load 
efficiency  of  this  group  is  very  nearly  .87.  The  efficiency  of 
the  system  is  then 

.94  x  .95  X  .87  =  77.6. 

This  result  requires  full  load  throughout  the  plant,  a  some- 
what unusual  condition  with  any  kind  of  distribution.  From 
the  data  already  given  the  half  load  efficiency  should  be  about 

•92  X  -975  X  .82  =  .735. 

Between  the  limits  just  computed  should  lie  the  commercial 
efficiency  of  any  well-designed  motor  distribution  reckoned 
from  the  dynamo  pulley.  In  the  case  of  steam-driven  plants 
it  is  often  desirable  to  consider  the  indicated  HP  of  the 
engine  as  the  starting  point,  and  the  question  immediately 
arises  as  to  the  commercial  efficiency  of  the  combination  of 
dynamo  and  engine.  In  cases  where  high  efficiency  is  the 
desideratum  direct  coupling  is  usually  employed,  saving 
thereby  the  loss  of  power,  perhaps  5  per  cent.,  produced  by 
belting.  The  losses  in  such  direct-coupled  units  vary  con- 
siderably with  the  size  and  type  of  both  machines.  Fig.  23 
shows  the  efficiency  of  two  such  combinations  at  various  loads. 
Curve  A  is  from  an  actual  test  of  the  combination  ;  curve  B 
from  tests  of  an  engine  and  dynamo  separately.  Each  unit 
was  of  several  hundred  HP.  The  high  result  from  curve  A  is 
mainly  due  to  very  low  friction. 


CONDITIONS  OF  POWER    TRANSMISSION. 


These  curves  give  handy  data  for  computing  the  total  effi- 
ciency of  a  motor  plant  from  the  motor  pulleys  to  the  indicated 
horse-power  of  the  driving  engine.  Taking  the  combined 
engine  and  dynamo  efficiency  from  A  and  assuming  the  same 
figures  as  before  on  motors  and  line  we  have  at  full  load 

.88  X    -95  X  -87  =  .727. 
And  from  the  same  data  at  half  load 

•78  X  -975  X  .82  =  .651. 

For  certain  computations,  as  in  case  of  figuring  out  a  com- 
plete installation,  the  above  efficiencies  are  convenient.  They 
show  that  in  very  many  instances  the  distribution  of  power  by 
electric  motors  is  very  much  more  economical  of  energy  than 
any  other  method  employed.  In  ordinary  manufacturing 
operations  power  is  generally  transmitted  to  the  working 
machines  through  the  medium  of  lines  of  shafting  oif  greater 
or  less  length.  These  are  very  rarely  belted  di»reot  to  the 


90 


CO 


FULL  LOAD 


FIG.  23. 


machines,  but  transfer  power  to  them  through  one  or  more 
countershafts.  Often  the  direction  of  shafts  is  changed  by 
gearing  or  quarter  turn  belts,  and  even  when  the  power  is 
distributed  through  only  a  single  large  building  there  will  be 


64  ELECTRIC    TRANSMISSION  OF  POWER. 

found  more  often  than  not  intervening  between  the  driving 
engine  and  the  driven  machine  three  belts  and  two  lines  of 
shafting  of  considerable  length,  and  not  infrequently  still  other 
belts  and  shafts.  It  very  often  happens,  too,  that  to  keep  in 
operation  one  small  machine  in  a  distant  part  of  the  shop  it  is 
necessary  to  drive  a  long  shaft  the  friction  of  which  consumes 
half  a  dozen  times  as  much  power  as  is  actually  needed  at  the 
machine.  The  constant  care  required  to  keep  long  lines  of 
shafting  in  operative  condition  is  an  irritating  and  costly  con- 
comitant.  The  necessary  result  is  a  considerable  loss  of 
power,  which  being  nearly  constant  in  amount  is  very  severe 
at  partial  loads. 

Allowing  5  per  cent,  loss  of  energy  for  each  transference 
of  power  by  belting,  a  figure  in  accordance  with  facts,  and  10 
per  cent,  loss  for  each*long  line  shaft  driven,  it  is  sufficiently 
evident  that  from  20  to  25  per  cent,  of  the  brake-horse-power 
delivered  by  the  engine  must  be  consumed  even  under  very 
favorable  circumstances  by  the  belting  and  shafting  at  full 
load.  This  means  an  efficiency  at  half  load  of  from  50  to  60 
per  cent,  only,  and  at  lesser  loads  a  very  low  efficiency  indeed. 

The  large  number  of  careful  experiments  carried  out 
on  shafting  in  different  kinds  of  workshops,  and  under 
various  conditions,  shows  that  only  under  very  exceptional 
circumstances  is  the  loss  of  power  by  shafting  between  the 
engine  and  the  driven  machines  as  low  as  25  per  cent.  Far 
more  often  it  is  from  30  to  50  per  cent.,  and  sometimes  as 
high  as  75  or  80  per  cent.  The  figures,  which  have  been  well 
established,  regarding  the  efficiency  of  the  transmission  of 
power  by  motors,  show  that  at  full  load  it  is  comparatively  easy 
to  surpass  75  per  cent,  efficiency;  thus  more  than  equaling  the 
very  best  results  that  can  be  obtained  with  shafting.  At  half 
load  and  below,  the  advantage  of  the  electric  transmission  be- 
comes enormous,  even  supposing  shafting  to  be  at  its  very  best. 

Compared  with  ordinary  transmission  by  shafting,  the  motor 
system  is  incomparably  superior  at  all  loads,  so  that  it  may 
easily  happen  that  a  given  amount  of  work  can  be  accomplished 
through  the  medium  of  a  motor  plant  with  one-half  the  steam 
power  required  for  the  delivery  of  the  same  power  through 
shafts  and  belts.  Such  results  as  this  have  actually  been 
obtained  in  practice.  It  is  therefore  safe  to  conclude  that  the 


CONDITIONS   OF  POWER    TRANSMISSION.  65 

distribution  of  power  by  motors  is,  under  any  ordinary  com- 
mercial conditions,  at  least  as  efficient  as  the  very  best  dis- 
tribution of  power  by  shafting  at  full  load  and  much  more 
efficient  at  low  loads.  Under  working  conditions  in  almost  all 
sorts  of  manufacturing  establishments,  light  loads  are  the  rule 
and  full  loads  the  rare  exception;  consequently  the  results  of 
displacing  shafting  by  motor  service  have,  as  a  rule,  been 
exceedingly  satisfactory  in  point  of  efficiency,  and  the  lessened 
operating  expense  more  than  offsets  the  extra  cost  of  instal- 
lation. 

In  one  large  three-phase  plant,  that  of  Escher,  Wyss  &  Co. 
at  Winterthur,  Switzerland,  300  HP  in  32  motors  worked  from 
a  i2-mile  transmission  line  displaced  far  greater  capacity  in 
steam  engines,  and  similar  results  on  a  smaller  scale  are  not 
uncommon. 

To  add  force  to  this  comparison  between  the  efficiency  of 
shafting  and  of  motors,  the  following  results  from  electrical 
distribution  plants  already  installed  may  be  pertinent.  One  of 
the  best  known  of  all  such  transmissions  is  that  at  the  fire- 
arms factory  at  Herstal,  Belgium.  There  are  there  installed 
17  motors  of  an  aggregate  capacity  of  305  HP,  driven  by  a  300- 
KW  generator  direct  coupled  to  a  500  HP  compound  condens- 
ing engine.  The  efficiency  guaranteed  from  the  shaft  of 
dynamo  to  the  pulleys  of  the  motors  is  77  percent.  Since  its 
first  installation,  the  plant  has  been  increased  by  the  addition 
of  a  second  direct-coupled  dynamo  and  the  total  horse- 
power of  motors  is  428.  A  second  notable  installation  of 
motors  in  the  same  vicinity  is  at  the  metallurgical  works  of 
La  Societe  de  la  Vielle-Montagne,  consisting  of  a  375  KW  500- 
volt  dynamo  direct  driven  at  a  speed  of  80  revolutions  per 
minute  by  a  600  HP  compound  condensing  engine.  The  plant 
consists  of  37  motors  with  an  aggregate  HP  of  329.  The  full 
load  efficiency  of  the  plant  from  dynamo  shaft  to  motor 
pulley  is  76  per  cent.  The  loss  in  the  lines,  both  in  this  case 
and  in  the  preceding,  is  very  small,  only  2  per  cent.  They 
are  both  typical  cases  of  transmission  to  motors  driving 
groups  of  machines,  and  in  spite  of  rather  low  dynamo  effi- 
ciencies at  full  load,  these  being  in  each  case  90  per  cent.,  the 
results  obtained  are  in  close  accordance  with  those  already 
stated  as  appropriate  to  similar  cases.  As  an  example  of 


66  ELECTRIC  TRANSMISSION  OF  FO  WER. 

work  under  somewhat  more  favorable  conditions,  the  three- 
phase  power  plant  at  Columbia,  S.  C,  may  be  instanced. 

The  problem  here  undertaken  was  to  drive  a  very  large  cot- 
ton mill,  utilizing  for  the  purpose  a  water  power  about  800  feet 
distant.  Two  5oo-KW  dynamos  direct  coupled  at  a  speed  of 
108  turns  per  minute  deliver  current  at  550  volts  to  an  under- 
ground line  connecting  the  power  station  with  the  mills. 
The  motors  are  suspended  from  the  ceiling  and  each  drives 
several  short  countershafts.  The  motors  are  wound  for  the 
generator  voltage  without  transformers,  and  are  of  a  uniform 
size,  65  HP  each.  The  commercial  efficiency  of  this  plant,  taken 
as  a  whole  from  the  shaft  of  the  dynamo  to  the  pulleys  of  the 
motors,  is  not  less  than  82  per  cent,  at  full  load.  This  good 
result  is  due  to  the  use  of  large  motors,  and  to  the  small 
line  loss  of  2  per  cent,  as  in  the  preceding  foreign  examples. 
These  results  are  thoroughly  typical,  and  can  regularly 
be  repeated  in  practice.  Even  smaller  plants  can  be  counted 
on  to  give  nearly  or  quite  as  good  results,  since  the  differ- 
ence in  efficiency,  supposing  motors  of  the  same  size  to  be 
used,  between  a  dynamo  of  100  KW  and  one  of  400  or  500  KW 
is  hardly  more  than  i  per  cent,  at  full  load,  supposing 
machines  of  the  same  general  design  to  be  employed,  nor  is 
there  any  substantial  difference  in  efficiency  between  plants 
employing  direct  current  and  those  using  polyphase  apparatus, 
as  may  be  judged  from  the  figures  just  given. 

We  are  now  in  position  intelligently  to  compare  the  trans- 
mission and  distribution  of  power  by  electric  means  with  the 
other  methods  which  have  sometimes  been  employed. 

All  comparisons  between  methods  of  transmitting  power 
have  to  be  based  in  a  measure  on  their  relative  efficiency. 
Now  in  every  such  method  there  are  three  essential  factors: 
ist,  the  generating  mechanism,  which  receives  power  direct 
from  the  prime  mover  and  in  conjunction  with  which  it  is 
considered;  2d,  the  transmitting  mechanism,  which  may  be  an 
electric  line,  a  pipe  line,  ropes,  or  belts,  and  3d,  the  motor 
part  of  the  transmission,  which  receives  power  from  the  trans- 
mitting mechanism  and  delivers  it  for  use.  For  a  given 
capacity  of  the  generating  and  receiving  mechanisms,  the 
efficiency  of  each  at  all  loads  is  determined  within  fairly  close 


CONDITIONS   OF  POWER    TRANSMISSION.  67 

limits.       The   transmitting    mechanism,    however,    is    not   so 
closely  determined,  save  in  the  case  of  the  rope  drive. 

Electric,  pneumatic  and  hydraulic  transmission  lines  are  all 
subject  to  the  general  principle  that  the  loss  in  transmission 
can  be  made  indefinitely  small  by  an  indefinitely  large  expen- 
diture of  capital,  enormous  cross-section  in  the  one  case  or 
huge  pipe  lines  in  the  others.  The  efficiency  of  these  methods 
is  therefore  a  fluctuating  quantity  depending  on  that  loss  in 
the  transmitting  mechanism  which  may  be  desirable  from  an 
engineering  or  economical  standpoint.  In  making  compar- 
isons between  these  methods,  there  is  a  wide  opportunity  for 
error  unless  some  common  basis  of  comparison  is  predeter- 
mined. In  the  next  case  any  such  comparison  must  differ 
widely  in  its  results  according  to  the  character  of  the  power 
distribution  which  is  to  be  attempted.  We  have  already  seen 
that  with  the  rope  drive  distribution  is  very  difficult,  while 
with  electric  and  pneumatic  systems  it  is  comparatively  easy. 

A  general  valuation  of  the  commercial  possibilities  of  these 
divers  matters  is  therefore  hard  to  make  except  in  a  general 
way  We  can,  however,  by  assuming  a  given  transmission  of 
given  magnitude  and  character,  and  further  assuming  such 
loss  in  the  transmitting  mechanism  as  might  reasonably  be 
expected  in  practice,  arrive  at  a  reasonably  accurate  conclu- 
sion for  the  case  considered.  As  a  very  simple  example  of 
power  transmission,  let  us  take  the  delivery  of  power  over  a 
distance  of  two  miles,  the  delivery  being  in  one  unit  or  at  most 
two  units.  WTe  will  assume  the  same  indicated  HP  furnished  at 
the  generating  end  of  the  line  in  each  case,  of  which  as  much  as 
possible  is  to  be  delivered  at  the  receiving  station,  the  losses 
in  transmission  being  taken  as  10  per  cent,  of  the  power 
delivered  to  the  line;  this  is  to  cover  all  losses  of  energy  by 
resistance  and  leakage  on  the  electrical  line  or  loss  of  pres- 
sure and  resulting  expenditure  of  energy,  leakage,  friction 
and  all  other  sources  of  loss  in  the  other  cases. 

As  the  same  indicated  power  is  generated  in  each  case,  we 
will  suppose  a  modern  plant  with  compound  condensing  en- 
gines costing  complete  with  buildings  $50  per  HP.  We  will 
further  assume  that  each  indicated  horse-power  per  working 
year  of  3,000  hours  will  cost  $18;  this  covering  all  expenses 
except  those  chargeable  to.  interest  and  depreciation.  For 


68  ELECTRIC   TRANSMISSION  OF  POWER. 

this  simple  case  we  have  the  following  costs  of  initial  plant 
and  of  operation  per  mechanical  horse-power  delivered  from 
the  motor,  full  load  only  being  considered.  The  four  meth- 
ods considered  are  rope  driving,  pneumatic,  pneumatic  with 
reheating  apparatus  at  the  motors,  and  electrical.  The  prices 
are  from  close  estimates  of  the  cost  in  each  case.  The  dyna- 
mos are  supposed  to  be  direct  coupled.  The  compressors  to 
t>e  direct  acting,  two-stage  compressors.  The  steam  cylinders 
Corliss  compound  condensing  type.  The  air  pressure  assumed 
is  60  Ibs.  above  atmospheric  pressure.  The  electric  voltage 
3,000.  The  rope  speed  about  one  mile  per  minute.  Interest 
and  depreciation  are  taken  at  10  per  cent,  of  the  total  cost  of 
the  plant,  save  in  the  case  of  the  rope  drive,  where  an  addi- 
tional charge  for  renewal  of  cable  is  made  on  the  supposition 
that  the  cable  will  last  somewhere  from  18  months  to  2  years, 
which  is  fully  as  favorable  a  result  as  can  fairly  be  expected. 
The  following  are  the  comparative  estimates: 


ROPE,  EFFICIENCY  67  PER  CENT. 

COST. 
Steam  plant,      .        .        .        «,        .        ;        ...      $50,000 

Pulley  stations,   '    .         . 25,000 

Cables,  steel,     .  17,000 

Total  cost,    . .: . '-•' $92,000 

OPERATING  EXPENSE. 

1,000  I. HP  at  $18, $18,000 

Interest  and  depreciation  on  plant,  at  10  per  cent,          .  7,5oo 

Depreciation  of  cable, 8,000 

$33,500 

Net  HP  produced,  672. 
Cost  per  HP-year,  $49. 


PNEUMATIC,  EFFICIENCY  54  PER  CENT. 

COST. 

Steam  plant  excluding  engines,       '    .        .        .        .  .      $35,000 

Compressors,          .         . 17,000 

Air  mains  laid,  12  inches,           .         .         .         .         .  .         18,000 

Air  motors, 12,000 

Total  cost,             ..oo..  $82,000 


CONDITIONS  OF  POWER    TRANSMISSION. 

OPERATING   EXPENSE. 


1,000  I.  HP  at  $18 

Interest  and  depreciation,  at  10  per  cent., 

Net  HP  delivered,  540. 
Cost  per  HP-year,  $48. 


$18,000 
8,200 

$26,200 


AIR  REHEATED,  APPARENT  EFFICIENCY  65  PER  CENT. 
COST. 

Steam  plant,  excluding  engines, $35,ooo 

Compressors,           .         .         .         .         .         .  '                .  17,000 

Air  mains  laid,            .•        ,  .-.     .         .         .         .         .         .  18,000 

Air  motors  and  reheaters  with  chimney,  etc.,           .         .  14,000 

Total  cost,    .                  $84,000 

OPERATING  EXPENSE. 

1,000 1.HP  at  $18,  .         . $18,000 

Interest  and  depreciation,  at  10  per  cent,         .         .         .  8,400 

Coal  and  labor  for  reheating r  COQ 

$27,900 

Net  HP  delivered,  650. 
Cost  per  HP  year,  $43. 

ELECTRIC,  EFFICIENCY  73  PER  CENT. 

COST. 

Steam  plant,      .         .         . :.    v        .         .         .;'.-.  $50,000 

Dynamos,       .       %.      .         ...       :.  -<     .         '.  -;,'.  18,000 

Line.          •         •      ...         .         .        7        .         .         .         .  3,000 

Motors>      .    -    ;    •        •        •        ,        .^    ;.         .        .  13,000 

Total  cost,           .         •    ,-   •         .-J.         .         .  $84,000 
OPERATING  EXPENSE. 

i.ooo  I. HP  at  $18,         .         .         .       _.         .    '     .         .  $18,000 

Interest  and  depreciation,  at  10  per  cent,             .  v     .        '•-  8,400 

Electrician,    .         .                  .         .....  1,500 


Net  HP,  730. 

Cost  per  HP-year,  $38. 


$27,900 


It  appears  at  once  that  the  rope  drive  is  beyond  the  range  of 
its  efficient  use.  Its  first  cost  is  greater  than  that  of  either  of 
the  other  methods  and  the  expense  is  carried  to  a  very  high 
figure  by  the  item  of  depreciation  on  the  cables,  which  cannot 


70  ELECTRIC  TRANSMISSION  OF  POWER. 

be  avoided,  hence  in  spite  of  a  high  efficiency  the  cost  per 
HP  year  delivered  rises  to  $49. ,  We  may  next  consider  the 
schedule  of  cost  for  the  pneumatic  system.  In  this  case  the 
most  formidable  item  is  the  cost  of  the  air  mains,  which  should 
beat  least  12  inches  in  diameter.  Nevertheless  the  total  initial 
cost  is  the  lowest  of  the  four.  The  operating  expense  is  also 
the  lowest,  but  the  very  low  efficiency  of  the  pneumatic  system 
without  reheating  raises  the  cost  per  HP  delivered  to  a  very 
considerable  amount;  almost  as  much  as  in  the  case  of  the 
rope  drive.  Reheating  would  almost  always  be  used  in  con- 
nection with  a  plant  of  this  size,  and  with  reheating  the  result 
is  much  more  favorable.  The  initial  expenditure  is  somewhat 
increased  by  the  addition  of  the  reheaters,  piping  and  chimney. 
The  operating  expense  is  also  slightly  increased  by  the  coal 
necessary  for  reheating,  taken  at  %  of  a  pound  per  HP  per 
hour  and  the  small  amount  of  additional  labor  involved  in 
caring  for  the  reheaters,  disposing  of  the  ashes  and  looking 
after  the  reheating  plant  generally.  The  apparent  efficiency 
in  this  case  is  very  excellent,  65  per  cent,  being  reasonably 
attainable,  and  the  cost  per  HP  year  falls  to  $43,  showing 
conclusively  enough  the  advantage  of  reheating;  at  least  where 
the  units  are  so  large  that  the  presence  of  a  reheater  is  not 
a  practical  nuisance. 

Finally,  we  come  to  the  electric  power  transmission.  In 
this  case  the  most  striking  feature  is  the  low  cost  of  the 
line,  supposed  here  to  be  overhead.  It  may  be  noted,  how- 
ever, that  an  underground  line,  consisting  of  cable  laid  in 
conduit,  still  leaves  the  cost  per  HP  year  lower  than  that 
of  any  of  the  other  methods.  Operating  expense  is  fairly 
increased  by  the  addition  of  an  electrician  to  the  cost 
of  the  indicated  horse  power,  interest  and  depreciation.  The 
total  first  cost  is  practically  the  same  as  that  of  air  with 
reheater,  as  is  also  the  operating  expense.  The  added  effi- 
ciency, however,  brings  the  cost  per  HP  year  to  $38;  decidedly 
the  lowest  of  the  four  cases  considered.  It  may  be  thought 
that  difference  of  loss  in  transmission  might  possibly  alter  the 
relation  of  the  electric  plant  to  the  air  plant  with  reheaters, 
but  an  added  efficiency  of  line  would  in  either  case  be  accom- 
panied by  added  expenditure  of  not  very  different  amounts  in 
the  two  cases,  and  the  efficiency  of  the  electric  plant  would 


CONDITIONS   OF  POWER    TRANSMISSION.  71 

always  be  enough  higher  than  that  of  the  air  plant  to* give  it 
the  advantage  in  net  cost  per  HP,  however  the  two  plants 
might  be  arranged.  We  thus  find  that  at  a  distance  of  two 
miles  the  electric  transmission  has  a  material  advantage,  air 
with  reheaters,  air  without  reheaters,  and  rope  drive  following 
it  in  the  order  named.  As  previously  mentioned  at  a  dis- 
tance of  one  mile  the  efficiency  of  the  rope  drive  is  so  far 
increased  as  to  bring  the  cost  per  HP  delivered  down  nearly 
to  that  of  the  electric  transmission.  The  pneumatic  method 
would  at  the  distance  of  one  mile,  as  may  readily  be  com- 
puted, take  about  the  same  relative  position  as  before,  since 
the  efficiency  of  the  two  maintains  approximately  the  same 
relation  to  the  others. 

The  pneumatic  plant  gains  in  first  cost  at  this  lesser 
distance,  not  enough,  however,  to  alter  the  final  result.  At 
half  a  mile  distance,  the  rope  drive  will  be  found  to  be  the 
cheapest  in  first  cost  and  also,  through  its  enormous  efficiency, 
to  be  the  cheapest  per  HP  delivered,  in  spite  of  the  large 
depreciation  in  the  cables,  while  the  electric  and  pneumatic 
systems  would  be  very  close  together,  the  electric,  however, 
still  retaining  a  slight  advantage  due  to  its  great  efficiency. 
Neither  can  in  point  of  absolute  cost  of  power  delivered  com- 
pete with  the  rope  drive  at  this  distance  for  this  large  and 
simple  transmission.  Figures  that  have  heretofore  been  given 
on  the  relative  cost  and  efficiency  of  such  transmissions  have  as 
a  rule  been  in  error  in  two  very  essential  particulars:  first,  the 
efficiencies  of  the  electrical  system  have  been  greatly  under- 
estimated owing  to  the  poor  machines  with  which  the  first 
experiments  were  made;  second,  the  commercial  advantage  of 
reheating  in  the  pneumatic  transmission  has  not  generally  been 
given  its  proper  weight.  It  is,  as  has  been  already  stated,  not 
a  method  of  increasing  the  efficiency  but  of  increasing  the 
power  delivered  by  addition  of  energy  at  the  receiving  end  of 
the  line  under  very  favorable  conditions.  The  figures  just 
given  are  believed  to  be  as  nearly  exact  as  present  conditions 
permit.  The  hydraulic  system  has  not  been  here  considered 
inasmuch  as  it  is  not  of  general  applicability. 

At  less  than  full  load  and  hence  under  variable  loads  the  elec- 
tric system  enjoys  the  unique  advantage  of  having  the  losses 
of  energy  in  every  part  of  the  system  decrease  as  the  load 


72 


ELECTRIC   TRANSMISSION-  OF  POWER. 


decreases,  while  in  rope  driving  all  the  losses  are  practically 
constant,  and  in  the  hydraulic  and  pneumatic  systems  all 
are  nearly  constant  save  that  in  the  pipe  line. 

Hence  under  low  and  varying  loads  electric  transmission  has 
a  great  additional  advantage.  Since  in  distributions  of  power 
employing  a  considerable  number  of  motors  light  load  on  the 
motors  is  the  invariable  rule,  as  soon  as  we  depart  from  the 
very  simple  case  discussed  the  electrical  system  gains  in  rela- 
tive economy  at  every  departure.  These  more  general  cases 
have  already  been  described,  and  gathering  the  results  we 
may  construct  the  following  table,  showing  the  efficiency  of 
each  system  under  full  and  half  loads: 


SYSTEM. 

FULL  LOAD. 

HALF  LOAD. 

\Vire  rope             .           

68 

46 

Hydraulic  high  pressure 

c-7 

Af 

Hydraulic  low  pressure                     

CQ 

CO 

Pneumatic                         .    ..        

CQ 

4O 

Pneumatic  reheated  (virtual  efficiency) 

fie 

CO 

Electric 

11 

6* 

ij 

The  efficiencies  in  the  electric  system  as  here  given  are 
lower  than  would  be  reached  practically  in  large  plants.  The 
present  practice  of  using  generators  and  motors  wound  for 
pressures  up  to  10,000  or  12,000  volts  makes  a  most  material 
difference  in  the  matter  of  efficiency.  For  a  straight  away 
transmission  of  a  few  miles  in  units  of  say  500  KW  and  up- 
wards, one  may  fairly  expect  to  get  at  full  load  as  much  as  94 
per  cent,  from  generator  and  motor,  and  perhaps  98  per  cent, 
from  the  line,  giving  a  total  efficiency  of  transmission  of 

.94  x    .94  X  .98  =  .866 

at  full  load  and  of  nearly  .85  at  half  load.  This  means  far 
higher  efficiency  than  can  be  obtained  by  any  other  method  at 
any  but  the  shortest  distances. 

All  the  figures  must  be  taken  as  approximate.  They  are 
under  conditions  fairly  comparable  except  in  case  of  the  low 
pressure  hydraulic  system,  in  which  the  large  proportion  of 
loss  due  to  pipe  friction  operates  to  hold  up  the  half  load  effi- 
ciency to  an  abnormal  degree.  With  the  ordinary  proportion 


CONDITIONS  OF  POWER    TRANSMISSION.  73 

of  small  motors  this  half  load  efficiency  would  be  nearer  40 
than  50  per  cent.  The  electric  system  is  easily  the  most 
efficient  at  any  and  all  loads.  Of  the  others,  wire  rope  trans- 
mission, if  the  distributed  units  are  fairly  large,  holds  the 
second  place  for  short  distances,  and  the  pneumatic  system 
with  energy  added  at  the  motors  by  reheating,  at  moderate 
and  long  distances.  Without  reheating  it  occupies  the  last 
place  in  order  of  efficiency,  although  even  so,  it  is,  next 
to  electricity,  the  most  convenient  method  of  distributing 
power. 

In  fact,  electricity  and  compressed  air  are  the  only  two 
systems  available  for  the  general  distribution  of  energy,  and 
also  the  most  used.  The  Popp  air  system  in  Paris  is,  save 
for  some  electric  central  stations,  the  largest  power  distribut- 
ing plant  in  the  world.  The  Edison  system  in  New  York  city 
has,  however,  above  10,000  HP  in  motors  operated  from  its 
mains,  and  other  similar  stations  have  loads  of  several  thou- 
sand HP.  Of  course  the  very  largest  power  stations  are 
those  belonging  to  electric  railway  systems  in  the  largest 
American  cities.  Several  of  these  exceed  25,000  HP  in 
generator  capacity  and  frequently  in  actual  output,  notably 
the  systems  in  Boston,  Brooklyn  and  Philadelphia.  Recent 
advances  in  electrical  engineering,  particularly  the  effective 
utilization  of  alternating  currents,  have  greatly  cheapened  the 
distribution  of  electrical  energy  and  other  systems  are  now 
seldom  installed  for  ordinary  purposes.  A  few  pneumatic  and 
hydraulic  plants  will  continue  to  be  used  owing  to  the  large 
capital  already  invested  in  them,  but  new  work  is,  and  in  the 
nature  of  things  must  be,  almost  exclusively  electrical.  As 
the  transmission  of  power  from  great  distances  becomes  more 
common  and  the  radii  of  distribution  themselves  increase,  the 
electrical  methods  gain  more  and  more  in  relative  value,  and 
all  others  become  more  inefficient  and  impracticable. 

We  have  now  discussed  in  some  detail  the  sources  of  natural 
energy  which  are  available  for  human  use,  and  the  most  promi- 
nent of  the  systems  employed  for  their  utilization.  We  have 
found  that  for  practical  purposes  steam  power  and  water 
power  must  at  present  be  used  to  the  virtual  exclusion  of  all 
others,  the  former  perhaps  less  than  the  latter  save  for  dis- 
tribution of  power  over  short  distances. 


74  ELECTRIC  TRANSMISSION  OF  POWER. 

Of  the  methods  of  distribution  we  have  found  all  save  com- 
pressed air  and  electricity  limited  in  their  application,  the 
hydraulic  systems  to  special  classes  of  work  under  favorable 
topographical  conditions,  and  rope  transmission  limited  to 
short  distances  and  small  numbers  of  power  units  delivered. 
Both  are  noticably  inefficient.  The  pneumatic  system  is  very 
general  in  its  applicability,  but  of  very  low  intrinsic  efficiency. 
When  used  in  connection  with  reheating  apparatus  it  requires 
additional  care,  and  the  motors  like  steam  engines  are  heavy 
and  inconvenient.  The  electric  system  on  the  other  hand  em- 
ploys motors  which  are  compact  and  far  more  efficient  than 
any  other  type  of  machine  for  delivering  mechanical  power, 
run  practically  without  attention,  and  can  be  placed  in  any 
situation  or  position  that  is  convenient.  Futhermore  in  aver- 
age working  efficiency  the  electric  system  is  10  to  15  per  cent, 
higher  than  any  other  yet  devised,  so  that  it  is  more  economical 
in  use  at  nearly  all  distances  and  under  nearly  all  conditions. 
Finally  it  unites  with  power  distribution  the  ability  to  furnish 
light  and  heat,  thus  gaining  an  immense  commercial  advan- 
tage. This  advantage  is  shared  only  by  gas  transmission, 
which  up  to  the  present  time  remains  of  doubtful  value  on 
account  of  the  cost  of  the  motors,  their  rapid  depreciation 
and  their  inefficiency  at  moderate  loads.  Having  now  over- 
looked its  advantages  in  general,  it  is  proper  to  pass  to  the 
details  of  the  methods  employed  for  its  utilization  and  thence 
to  the  general  problem  of  its  economical  generation,  trans- 
mission and  distribution. 


CHAPTER  III. 

POWER    TRANSMISSION    BY    CONTINUOUS    CURRENTS. 

UP  to  the  present  time  by  far  the  largest  part  of  electrical 
power  transmission  has  been  done  by  continuous  currents. 
All  the  earlier  plants  were  of  this  type,  and  even  now,  when 
transmission  by  alternating  currents,  polyphase  and  other,  is 
pushing  rapidly  to  the  front,  the  older  type  of  apparatus  is 
still  being  installed  on  an  extensive  scale,  and  on  account  of 
the  large  number  of  plants  now  in  operation,  even  if  for  no 
other  reason,  will  probably  remain  in  use  for  a  long  time  to 
come.  New  power  transmission  plants,  both  here  and  abroad, 
are  more  and  more  frequently  installed  for  alternating  cur- 
rents, and  in  many  cases  this  practice  is  almost  absolutely 
necessary,  but  there  still  remain  many  cases  wherein  the  con- 
ditions are  as  well  or  better  met  in  the  old-fashioned  way. 

Chief  among  these  may  be  mentioned  electric  railway  work, 
which  in  America  alone  probably  requires  more  than  a  full 
million  horse  power  in  generators  and  motors.  Certain  diffi- 
cult work  at  variable  speed  and  load,  and  many  simple  trans- 
missions over  short  distances,  are  at  present  best  handled  by 
continuous  current  machinery.  As  alternating  practice  ad- 
vances many,  perhaps  all,  of  these  special  cases  will  be  elimin- 
ated, but  we  are  dealing  with  the  art  of  power  transmission  as 
it  exists  to-day,  and  hence  continuous  current  working  deserves 
very  careful  consideration. 

The  broad  principle  of  the  continuous  current  generator  has 
already  been  explained,  but  its  modifications  in  actual  work 
are  important  and  worthy  of  special  investigation.  In  a  gen- 
eral way,  continuous  currents  are  almost  always  obtained  by 
commuting  the  current  obtained  from  a  machine  which  would 
naturally  deliver  alternating  currents.  This  process  is,  how- 
ever, by  no  means  as  simple  as  Fig.  9  would  suggest.  With  a 
two-part  commutator  the  resulting  current,  although  unidirec- 
tional, would  necessarily  be  very  irregular  owing  to  the  fact 

75 


76 


ELECTRIC   TRANSMISSION  OF  POWER. 


that  the  total  current  drops  to  zero  at  the  moment  of  com- 
mutation. Such  a  current  is  ill  fitted  for  many  purposes,  and 
the  commutator  would  be  rapidly  destroyed  by  sparking  if  the 
machine  were  of  any  practical  size. 

To  avoid  these  difficulties,  the  number  of  coils  on  the  arma- 
ture is  increased,  and  they  are  so  interconnected  that,  while 
each  coil  has  its  connection  to  the  outside  current  reversed  as 
before,  when  its  electromotive  force  is  zero,  the  other  coils  in 
which  the  E.  M.  F.  still  remains  in  the  right  direction  continue 
in  circuit  unchanged.  In  this  way  the  E.  M.  F.  at  the  brush 
is  the  sum  of  the  E.  M.  Fs.  of  a  number  of  coils,  each  of  which 
is  reversed  at  the  proper  moment.  The  number  of  commutator 
segments  is  increased  proportionally  to  the  number  of  coils, 


FIG.  24. 

and  the  commutator  thus  becomes  a  comparatively  complicated 
structure.  The  result,  however,  is  that  the  total  E.  M.  F.  of 
the  armature  cannot  vary  by  more  than  the  variation  due  to  a 
single  coil.  The  nature  of  this  modification  is  shown  in  Fig. 
24,  which  shows  a  four-part  commutator  connected  to  a  four- 
coil  drum  armature. 

An  eight-part  winding  of  modern  type  is  shown  in  Fig.  25. 
Tracing  out  the  currents  in  this  will  give  a  clear  idea  both  of 
a  typical  winding  and  of  the  process  of  commutation. 

In  commercial  machines  the  number  of  individual  coils  and 
of  commutator  segments  often  exceeds  100,  but  the  principle 
of  the  winding  is  the  same.  Nearly  all  the  early  dynamos  had 
several  turns  of  wire  per  coil,  as  in  Fig.  24,  but  at  present,  in 
most  large  machines,  one  turn  constitutes  a  complete  coil. 
This  extreme  subdivision  is  to  avoid  sparking  at  the  commu- 


POWER  TRANSMISSION  BY  CONTINUOUS   CURRENTS.       77 

tator,  which  becomes  destructive  if  the  current  be  large  and 
the  E.  M.  F.  per  commutator  segment  great. 

If  each  coil  generates  a  considerable  voltage,  there  is  even 
under  the  best  conditions  of  commutation  a  strong  tendency 
for  sparks  to  follow  the  brush  across  the  insulation  between 
segments,  or  even  to  jump  across  this  insulation  elsewhere. 
As  this  goes  from  bad  to  worse  and  rapidly  ruins  the  commu- 
tator, every  precaution  has  to  be  taken  against  such  a  con- 
tingency. The  E.  M.  F.  generated  by  each  coil  is  kept  low 
by  subdividing  the  winding,  and  in  large  machines  it  is  the 


FIG. 25. 


rule  that  the  E.  M.  F.  of  a  single  loop  is  quite  all  that  can 
safely  be  allotted  to  a  single  commutator  segment. 

Present  good  practice  indicates  that  for  generators  for  light- 
ing up  to  100  or  150  volts  the  voltage  between  brushes  should 
be  subdivided  so  that  it  shall  not  exceed  3  or  4  volts  for 
each  segment  between  the  brushes.  For  500  or  600  volt 
machines  it  should  not  ordinarily  exceed  about  10  volts,  while 
for  dynamos  of  moderate  output  and  even  higher  voltage  it 
may  rise  to  20  volts  or  more. 

The  reason  for  these  different  figures  is  that  the  destructive- 
ness  of  the  spark  depends  on  the  amount  of  current  which  is 


78  ELECTRIC    TRANSMISSION  OF  POWER. 

liable  to  be  involved.  On  a  low  voltage  commutator  intended 
for  heavy  currents,  even  very  moderate  sparking  may  gnaw 
the  segments  seriously,  while  the  spark  of  an  arc  machine  in 
spite  of  its  venomous  appearance  may  do  very  little  harm,  as 
the  maximum  current  in  the  whole  bar  will  not  exceed  8  or  10 
amperes.  Consequently  the  voltage  per  bar  in  such  cases  is 
sometimes  50  or  more,  while  in  very  large  incandescent  ma- 
chines and  in  those  designed  for  electrolytic  purposes  the  E. 
M.  F.  per  bar  is  often  less  than  2  volts  or  even  below  i  volt. 


FIG.  26. 

Windings  like  those  of  Figs.  24  and  25  are  of  the  so-called 
drum  type,  in  which  each  convolution  extends  around  the  whole 
body  of  the  armature,  either  diametrically  or  nearly  so. 
Another  sort  of  armature  winding  frequently  used,  although 
less  now  than  formerly,  is  the  Gramme,  so  called  from  its 
inventor.  Here  the  iron  body  of  the  armature  is,  instead 
of  being  cylindrical,  in  the  form  of  a  massive  ring  of  rect- 
angular cross  section.  The  windings  are  looped  through  and 
around  this  ring,  fitting  it  firmly  and  closely.  Fig.  26,  which 
shows  in  diagram  a  winding  in  ten  sections,  furnishes  a  good 
example  of  the  Gramme  construction.  There  may  be  one  or 
several  turns  per  coil,  as  in  drum  windings.  These  two  gen- 
eral types  of  windings  are  used  with  various  modifications  in 
nearly  all  continuous  current  dynamos.  Each  has  its  good 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.         79 

and  bad  features.  The  Gramme  winding  makes  it  very  easy 
to  keep  down  the  voltage  per  segment,  inasmuch  as  for  each 
external  armature  wire  there  is  a  commutator  bar,  while  in 
the  drum  form  there  is  but  one  bar  for  two  wires.  It  is  also 
mechanically  solid  even  when  wound  with  small  wire,  and  no 
two  adjacent  wires  can  have  a  considerable  voltage  between 
them,  thus  making  it  easy  to  build  an  armature  for  high  E. 
M.  F.  On  the  other  hand,  the  drum  winding  gives  a  very 
compact  armature  of  easy  construction,  and  the  magnetism 
induced  in  it  is  less  likely  to  disturb  that  of  the  field. 

In  the  small  machines  once  usual  the  Gramme  type  was  pre- 
ferred for  high  voltages  on  account  of  the  ease  with  which  it 
could  be  repaired,  while  the  drum  was  liked  for  its  simplicity 
of  mechanical  construction  as  a  whole  and  excellent  efficiency 
as  an  inductor.  In  modern  practice  the  differences  between 
these  types  have  become  much  less  marked.  With  large  units, 
particularly  of  the  multipolar  form  now  usual,  the  drum  wind- 
ing is  as  easily  insulated  as  the  Gramme,  for  with  the  winding 
now  used  in  such  cases  there  need  be  no  considerable  voltage 
between  adjacent  wires,  and  repairs  are  of  very  infrequent 
occurrence.  In  fact,  the  drum  winding  can  be  made  quite  as 
accessible  as  the  other,  and  is  on  the  whole  cheaper  and 
simpler.  Almost  the  sole  advantage  of  the  Gramme  (or  ring) 
winding  is  that  of  low  voltage  per  commutator  bar.  Mechanic- 
ally, too,  there  is  less  difference  than  formerly,  for  the  coils 
are  in  both  types  frequently  bedded  in  slots  in  the  iron  of  the 
armature  core. 

It  must  be  noted  that  the  armature  of  the  modern 
dynamo,  unless  of  small  size  or  unusually  high  voltage,  is  sel- 
dom wound  with  wire  in  the  ordinary  sense  of  the  word.  In- 
stead, the  conductors  are  bars  of  copper,  usually  of  sections 
rectangular  rather  than  round,  and  generally  lacking  any  per- 
manently attached  insulation.  Whatever  the  winding,  the 
conductors  on  the  armature  face  are  inclosed  in  close  fitting 
tubes  of  mica  and  specially  treated  paper  or  the  like,  and  then 
put  in  place  on  the  armature  core  or  in  more  or  less  completely 
closed  channels  cut  in  it.  If  on  the  core  surface,  the  bars  are 
often  not  insulated  on  the  exterior  surface  at  all.  If  the  arma- 
ture core  be  slotted,  the  insulating  material  is  preferably  put 
in  position  first  and  the  bar  put  in  afterward.  As  to  the  rest 


8o  ELECTRIC   TRANSMISSION  OF  POWER. 

of  the  winding  it  is  completed  by  connectors  of  copper  strip  or 
rod  soldered  to  the  face  conductors  and  insulated  in  a  substan- 
tial manner.  Thus  each  convolution,  whether  of  ring  or  drum 
winding,  is  composed  of  from  two  to  four  pieces. 

A  typical  modern  ring  winding  is  shown  in  Fig.  27.  It  well 
exemplifies  the  construction  above  mentioned,  and  in  this  case 
the  uninsulated  faces  of  the  exterior  conductors  form  the  com- 
mutator of  the  machine.  Such  a  construction  of  course  ex- 
cludes iron  clad  armatures  and  is  best  fitted  for  a  machine 
having  a  field  magnet  inside  the  ring  armature.  A  similar 


FIG.  27. 

arrangement  which  avoids  the  above  limitations,  uses  the  side 
connectors  of  the  ring  as  commutator  segments.  The  general 
principle,  however,  is  the  same,  whether  the  commutator 
forms  part  of  the  winding  proper  or  is  a  separate  structure. 

An  iron  clad  drum  winding  of  typical  character  is  shown  in 
Fig.  28.  Here  the  exterior  bars  are  fitted  into  thoroughly 
insulated  slots  in  the  core,  and  wedged  firmly  into  place  by 
insulating  wedges.  Sometimes  the  bars  themselves  are  shaped 
so  as  to  act  as  wedges.  In  either  case  the  bars  are  held  almost 
as  solidly  as  if  they  formed  an  integral  part  of  the  core.  The 
commutator  in  these  windings  must  be  a  separate  affair.  Fig. 


POWER  TRANSMISSION  BY  CONTINUOUS  CURRENTS.        81 

28  shows  well  the  nature  of  the  winding,  with  its  slotted  core, 
ventilating  spaces,  and  massive  bars  — in  this  example  4  per 
slot.  The  end  connectors  lie  in  a  pair  of  reverse  spirals,  one 
outside  the  other,  and  separated  by  firm  insulation.  The 
relation  of  these  connectors  to  the  rest  of  the  winding  is  illus- 
trated in  Fig.  25. 

Between  the  modern  drum  and  ring  armatures  it  is  difficult 
to  discriminate.  Both  have  been  successfully  used  in  dynamos 
of  the  largest  size,  but  the  iron  clad  drum  is  in  the  more  gen- 
eral use,  while  the  use  of  ring  armatures  is  declining.  It  is 
rather  unusual  to  find  a  standard  generator  of  recent  build  of 
100  KW  or  more  output  with  a  regular  wire  wound  armature, 
and  the  most  of  them  have  some  modification  of  the  bar  wind- 
ings just  described. 


FIG.  28. 

We  have  briefly  reviewed  here  the  armature  windings  at 
present  in  general  use  and  may  now  pass  to  the  various  wind- 
ings employed  for  the  field  magnets.  These  are,  in  continuous 
current  dynamos,  almost  always  connected  with,  and  supplied 
with  current  from,  the  armature  winding,  thus  making  the 
machines  self-exciting.  As  the  armature  is  turned  the  action 
begins  with  the  weak  residual  magnetism  left  in  the  field  mag- 
nets, and  the  current  set  up  by  the  small  E.  M.  F.  thus  produced 
is  passed  around  and  gradually  strengthens  the  magnets,  build- 
ing them  up  to  full  strength.  If  this  residual  magnetism  is 
very  feeble,  as  may  happen  when  it  is  knocked  out  of  the  iron 
by  rough  handling  or  the  continual  jarring  of  a  long  journey, 
it  is  sometimes  quite  difficult  to  get  the  machine  into  action. 

The  simplest  form  of  field  winding,  and  the  one  which  was 
most  extensively  used  at  first,  is  that  in  which  the  current  from 


82 


ELECTRIC   TRANSMISSION  OF  POWER. 


one  of  the  brushes  passes  around  the  field  magnet  coils  on  its 
way  to  or  from  the  external  circuit  of  the  machine,  as  shown 
in  Fig.  29.  This  series  winding  possesses  more  than  one  ad- 
vantage. It  consists  of  a  comparatively  small  number  of  con- 
volutions of  rather  large  wire  and  so  is  cheap  to  wind;  it  is, 
for  this  same  reason,  little  liable  to  injury  and  easy  to  repair 
when  injured;  and  what  is  of  particular  importance,  whenever 
the  series  dynamo  is  called  upon  for  more  current,  the  mag- 
netizing power  of  the  field  is  raised  by  the  increase,  thus 
increasing  the  electromotive  force.  This  property,  once  con- 
sidered a  disadvantage,  becomes  of  great  value  in  modern 


FIGS.  29  AND  30. 

windings  adapted  for  the  purpose.  As  the  generation  of 
E.  M.  F.  at  the  start  depends  entirely  on  the  residual  mag- 
netism, series  wound  machines  do  not  "  build  up  "  full  voltage 
very  easily  unless  the  resistance  of  the  outside  circuit  is  fairly 
low,  thus  giving  the  current  a  chance. 

The  common  shunt  winding  shown  in  Fig.  30  almost  describes 
itself.  The  brushes  are,  independently  of  the  exterior  circuit, 
connected  to  magnetizing  coils  of  fine  wire.  Although  such  a 
field  winding  is  slightly  harder  to  construct  and  to  maintain,  it 
produces  a  magnetic  field  that  is  relatively  free  from  any  actions 
in  the  working  circuit  of  the  machine.  So  long  as  the  E.M.  F. 
at  the  brushes  is  unaffected  by  changes  of  speed,  the  field  will 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.        83 

be  quite  steady  except  as  a  very  large  current  in  the  exterior 
circuit  may  reduce  the  voltage  available  for  the  field  by  causing 
a  loss  of  voltage  in  the  armature.  If  the  armature  resistance 
be  very  small,  there  will  be  almost  a  constant  E.  M.  F.  at  the 
brushes  except  as  the  current  flowing  in  the  armature  may 
produce  a  magnetization  opposed  to  the  shunt  field.  For  a 
considerable  time,  then,  the  shunt  winding  was  always  used 
when  a  constant  E.  M.  F.  was  required.  At  the  same  time,  it 
permits  the  E.  M.  F.  to  be  varied,  if  desired,  with  a  very  small 
loss  of  energy,  by  the  simple  expedient  of  putting  a  variable 
resistance  in  circuit  with  the  field  magnets. 

As  the  principles  of  dynamo  construction  became  better 
known,  it  was  apparent  that  the  above  method  of  getting  a 
constant  E.  M.  F.  was  rather  expensive.  To  build  an  armature 
that  would  carry  a  heavy  current  without  noticeable  loss  of 
voltage  and  to  inclose  it  in  fields  so  strong  as  to  be  disturbed 
only  in  a  minute  degree  by  the  magnetizing  effects  of  such 
currents,  was  a  task  requiring  much  care  and  a  great  amount  of 
material.  Even  if  this  difficult  problem  were  solved,  the  con- 
stant voltage  would  be  at  the  brushes  of  the  machine  and  not 
at  the  load,  where  it  is  needed. 

An  easy  way  out  of  these  difficulties  is  found  by  considering 
an  important  property  of  the  series-wound  machine  just  men- 
tioned, /.  <?.,  the  rise  of  E.  M.  F.  as  the  load  on  the  external 
circuit  rises.  If  now  one  takes  a  good  shunt-wound  dynamo 
and  adds  to  the  field  magnets  a  few  series  turns  wound  in  the 
same  direction  as  the  shunt,  the  result  is  as  follows:  At  no- 
load,  the  voltage  at  the  brushes  is  that  due  to  the  shunt  alone. 
As  the  load  comes  on  this  voltage  would  naturally  fall  off  by  the 
loss  of  voltage  from  armature  resistance  and  reaction.  The 
series  turns,  however,  at  this  juncture  strengthen  the  field  and 
thus  compensate  for  these  losses.  This  is  the  compound 
winding  now  very  generally  used.  It  is  shown  in  diagram  in 
Fig.  31.  Ordinarily  the  series  turns  are  more  than  would  be 
needed  for  merely  compensating  the  losses  due  to  armature 
resistance  and  reaction,  so  that  the  voltage  at  the  brushes 
under  load  will  rise  enough  to  make  up  for  the  increased  loss 
in  the  line  due  to  carrying  heavier  current. 

Machines  thus  over-compounded  five  or  ten  per  cent,  are  in 
very  common  use. 


$4  ELECTRIC   TRANSMISSION  OF  POWER. 

The  foregoing  gives  the  rudiments  of  the  machines  used  for 
generating  direct  current.  It  now  remains,  before  taking  up 
the  question  of  power  transmission  proper,  to  consider  briefly 
the  use  of  such  machines  as  motors.  The  underlying  principle 
has  been  already  discussed.  The  power  of  a  motor  to  do 
work  depends  on  the  stress  of  the  magnetic  field  on  conductors 
carrying  current  in  it  and  free  to  move.  This  stress  is 
virtually  the  same  as  that  which  has  to  be  overcome  in  using 
the  machine  as  a  generator,  and  reaches  a  very  considerable 
amount  in  machines  of  any  size. 


FIG.  31. 

In  motors  with  the  field  strengths  often  used,  the  actual 
drag  between  the  field  and  the  armature  wires  may  amount  at  a 
rough  approximation  to  nearly  an  ounce  pull  on  each  foot  of 
conductor  in  the  field  for  every  ampere  flowing  through  the 
wire.  With  a  20  HP  motor  the  actual  twisting  effort  or  torque 
at  the  surface  of  the  armature  might  easily  be  considerably 
over  a  hundred  pounds  pull.  Forces  of  this  size  emphasize 
the  need  of  solid  armature  construction,  with  the  conduc- 
tors firmly  locked  in  place,  particularly  since  the  magnetic 
drag  is  not  steady,  but  comes  somewhat  violently  upon  the 
conductors  as  they  enter  the  field.  With  the  old  smooth  core 
armatures  wound  with  wire,  the  conductors  not  infrequently 


POWER   TRANSMISSION-  BY  CONTINUOUS  CURRENTS.        85 

worked  loose  and  chafed  each  other,  and  even  the  entire  wind- 
ing has  been  known  to  slip  on  the  core.  In  modern  windings, 
either  iron  clad  or  modified  smooth  core,  such  accidents  are 
nearly  impossible. 

When  the  armature  conductors  of  the  motor  cut  through  its 
field  as  the  armature  revolves,  an  electromotive  force  is  neces- 
sarily generated  in  them  as  in  every  other  case  when  the 
magnetic  forces  on  a  conductor  change.  There  is  thus  pro- 
duced, as  a  necessary  part  of  the  action  of  every  motor,  a 
counter  electromotive  force  in  the  armature.  This  electro- 
motive force  plays  a  very  important  part  in  the  internal 
economy  of  the  motor  and  is  worth  looking  into. 

In  the  first  place,  the  magnitude  of  the  counter  electromo- 
tive force  determines  the  amount  of  current  that  can  flow 
through  the  motor  when  supplied  at  a  given  voltage.  The 
resistance  of  the  armature  measured  from  brush  to  brush  may 
be  only  a  few  thousandths  or  even  ten  thousandths  of  an  ohm, 
while  the  applied  voltage  may  be  several  hundred  volts.  The 
current,  however,  is  not  that  which  would  flow  through  the 
given  resistance  under  the  pressure  applied,  but  the  flow  is 
determined  by  the  difference  between  the  applied  electro- 
motive force  and  the  counter  E.  M.  F.  of  the  motor,  so  that 
in  starting  a  mjtor  when  the  armature  is  at  rest  and  there  is 
therefore  no  counter  E.  M.  F.,  a  resistance  must  be  inserted 
outside  the  armature  to  cut  down  the  initial  rush  of  current. 

In  the  second  place,  the  counter  electromotive  force  meas- 
ures the  output  of  the  motor  for  any  given  current.  It  does 
this  because  the  very  same  things,  /.  *?.,  strength  of  field, 
amount  of  wire  under  induction,  and  speed,  which  determine 
the  output  for  a  given  current,  also  determine  the  magnitude 
of  the  counter  electromotive  force. 

Therefore,  when  the  machine  is  running  as  a  motor,  while 
the  energy  supplied  to  it  is  the  product  of  the  voltage  by  the 
amperes  which  flow  through  the  armature,  the  output  of  the 
motor  is  determined  by  the  product  of  the  counter  electromo- 
tive force  into  the  selfsame  current;  hence,  under  given 
conditions,  the  ratio  between  the  impressed  and  counter 
electromotive  forces  of  the  motor  determines  the  efficiency  of 
the  motor.  The  difference  between  these  electromotive  forces 
determines  the  input  of  energy,  since  it  determines  the  cur- 


86  ELECTRIC   TRANSMISSION  OF  POWER. 

rent  which  may  flow;  therefore,  as  the  counter  electromotive 
force  increases,  the  efficiency  of  the  motor  increases,  but  the 
output  is  limited  by  the  decreased  input. 

With  a  fixed  electromotive  force  supplied  to  the  armature, 
the  output  of  the  motor  per  ampere  of  current  will  diminish  as 
the  counter  electromotive  force  diminishes,  but  the  total 
amperes  flowing  will  increase  because  the  difference  between 
the  applied  and  counter  E.  M.  F.  has  also  increased.  Thus, 
the  total  output  increases,  although  at  a  lower  efficiency^ 
when  the  counter  E.  M.  F.  decreases.  Since  the  input  (which 
is  determined  by  the  difference  between  counter  and  applied 
E.  M.  Fs.)  multiplied  by  the  efficiency  (which  is  determined 
by  the  counter  E.  M.  F.)  equals  the  net  output  of  the  motor, 
this  output  will  be  at  a  maximum  when  the  counter  E.  M.  F. 
and  the  effective  E.  M.  F.  are  equal  to  each  other.  This  fol- 
lows from  the  general  law,  that  the  product  of  two  quantities, 
the  sum  of  which  is  fixed,  will  be  a  maximum  when  these 
quantities  are  equal. 

It  must  be  distinctly  understood,  however,  that  at  this  point 
of  theoretical  maximum  output  the  motor  is  very  inefficient, 
and  that  mechanical  considerations  prevent  the  efficiency 
being  wholly  determined  by  the  counter  E.  M.  F.,  while 
sparking  and  heating  generally  prevent  working  with  the 
counter  E.  M.  F.  equal  to  the  effective  E.  M.  F. 

In  actual  practice  motors  are  worked  under  very  diverse 
conditions,  and  some  of  these  it  is  worth  while  to  take  up  in 
detail,  following  the  preceding  generalizations.  The  energy 
may  be  supplied  at  constant  current,  at  constant  voltage,  with 
neither  current  nor  voltage  constant,  at  fixed  or  variable 
speed,  and  subject  to  a  wide  variety  of  conditions  ;  the  motors- 
may  be  wound,  either  series,  shunt,  compound,  or  with  various- 
modifications  of  these  windings,  and  may  be  either  self  regu- 
lating with  respect  to  various  requirements,  or  regulated  by 
extraneous  means.  In  the  ordinary  problems  dealt  with  in 
power  transmission,  these  conditions  may  be  classified  in  a 
fairly  simple  way  as  follows: 

Case  I.   Series-wound  motors  at  constant  current. 

Case  II.   Series-wound  motors  at  constant  voltage. 

Case  III.  Series-wound  motors  with  interdependent  current 
and  voltage. 


POWER   TRANSMISSION  BY  CONTINUOUS   CURRENTS.        87 

Case  IV.   Shunt-wound  motors  at  constant  voltage. 

The  first  class  is  now  less  widely  used  than  formerly, 
and  is  of  great  practical  importance  only  in  a  few  cases. 
The  second  class  is  very  widely  used  in  a  particular  case,  to 
wit:  electric  railway  practice,  and  consequently  it  is  of  great 
importance.  The  third  class  of  motors  is  used  occasionally 
with  great  success  but  not  very  extensively,  while  the  fourth 
includes  the  vast  majority  of  all  the  machines  running  for  pur- 
poses other  than  electric  railway  service.  These  cases,  there- 
fore, it  is  worth  while  to  take  up  somewhat  thoroughly. 

CASE  I. — Series-wound  motors  operated  with  a  constant  cur- 
rent originally  came  into  use  in  connection  with  arc  lighting 
circuits,  which  for  some  years  formed  the  most  generally 
available  source  of  current.  Such  lines  are  fed  from  dynamos 
in  which  the  current  is  kept  constant  by  special  regulation, 
while  the  voltage  rises  and  falls  in  accordance  with  the  load, 
consisting  of  lamps  or  motors  in  series  with  each  other.  We 
are  therefore  relieved  of  any  concern  about  the  current,  since 
it  is  kept  constant  quite  irrespective  of  what  happens  in  the 
motor. 

Under  these  circumstances,  in  a  series-wound  motor,  the 
torque  will  be  constant,  since  the  field  is  constant,  and  the 
output  of  the  motor  will  vary  directly  with  the  speed.  If  it 
be  loaded  beyond  its  capacity,  it  simply  refuses  to  start  the 
load,  inasmuch  as  its  torque  is  limited  by  the  current.  If  it 
starts  with  a  load  within  its  limit  of  torque,  its  speed  will 
steadily  increase  until  that  limit  is  reached.  This  may  be 
comparatively  soon  if  the  load  is  a  rapidly  increasing  one,  or 
the  machine  may  race  until  its  own  friction  of  air  and  bearings, 
magnetic  resistances  and  the  induction  of  idle  currents  in  the 
core  and  frame  serve  to  furnish  resistance  up  to  its  limit  of 
torque.  When  running  at  a  given  speed,  any  increase  of  load 
causes  the  speed  to  fall  off,  while  decrease  of  load  produces 
racing.  Unless  these  tendencies  are  controlled,  this  type  of 
machine  becomes  almost  useless  for  practical  purposes,  as 
regularity  of  speed  under  change  of  load  is  generally  highly 
desirable.  In  fact,  the  tendency  to  run  at  constant  torque  is 
generally  inconvenient.  To  obviate  this  very  serious  difficulty 
various  devices  have  been  tried  with  tolerable  success.  The 
commonest  is  to  vary  the  torque  in  accordance  with  the  load 


88  ELECTRIC   TRANSMISSION  OF  POWER. 

by  changing  the  field  strength  or  by  shifting  the  brushes  so  as 
to  throw  the  armature  coils  out  of  their  normal  relation  to  the 
magnetic  field. 

Since  the  object  of  such  changes  is  to  vary  the  output  at 
constant  current,  and  since  this  output  is  measured  by  the 
counter  E.  M.  F.  of  the  motor,  the  real  problem  of  such  regu- 
lation is  to  vary  the  counter  E.  M.  F.  in  proportion  to  the 
output  desired.  Therefore  the  same  general  means  that 
serve  to  accomplish  this  end  in  an  arc  dynamo,  keeping  the 
current  constant  and  varying  the  E.  M.  F.,  will  serve  to  regu- 
late the  corresponding  motor. 


As  in  this  case  the  speed  is  the  thing  to  be  held  constant, 
the  usual  means  taken  for  working  the  regulating  devices  is  a 
centrifugal  governor,  which  generally  acts  to  shift  the  brushes 
or  to  put  in  circuit  more  or  less  of  the  field  winding,  which  for 
this  purpose  is  divided  into  sections.  In  still  other  arrange- 
ments the  governor  acts  to  slide  the  armature  partially  into  or 
out  of  the  field,  or  to  work  a  rheostat  which  shunts  the  field 
magnet,  as  in  the  Brush  regulator  for  constant  current.  An 
excellent  example  of  a  small  constant  current  motor  regulated 
on  the  last  mentioned  principle  is  shown  in  Fig.  32. 

As  to  the  operation  of  these  regulating  devices,  it  is  tolerably 
good  if  everything  is  carefully  looked  after  and  kept  in  adjust- 


POWER  TRANSMISSION'  BY  CONTINUOUS  CURRENTS.         89 

ment.  The  efficiency  of  such  motors  is  not  generally  as  high 
as  that  of  other  types  at  light  loads,  owing  to  the  nearly  con- 
stant loss  in  the  armature  due  to  constant  current  working. 
At  and  near  full  load  the  efficiency  may  be  good. 

In  addition,  the  current  is  highly  dangerous,  coming  as  it  does 
from  generators  of  very  high  voltage,  and  even  the  voltage 
across  the  brushes  is,  in  machines  of  any  size,  sufficient  to 
give  a  dangerous  or  even  fatal  shock.  A  10  HP  motor,  for 
example,  on  the  customary  lo-ampere  circuit,  would  have  a 
difference  of  potential  of  about  800  volts  between  the  brushes 
at  full  load.  As  a  few  such  motors  would  load  even  the  largest 
arc  dynamos,  besides  being  dangerous  in  themselves,  opera- 
tions have  generally  been  confined  to  smaller  units.  On 
account  of  the  danger  and  the  mechanical  and  other  difficulties, 
the  arc  motor  has  come  to  be  looked  upon  as  a  last  resort,  is 
seldom  or  never  used  when  anything  else  is  available,  and,  to 
the  credit  of  the  various  manufacturers  be  it  said,  is  nearly 
always  sold  and  installed  with  a  specific  explanation  of  its 
general  character  and  the  precautions  that  must  be  taken 
with  it. 

In  spite  of  all  these  objections,  the  constant  current  motor 
often  does  good  and  steady  work,  and  some  such  motors  have 
been  used  for  years  without  accident  or  serious  trouble  of  any 
kind.  They  have  been  employed,  however,  only  sparingly  for 
power  transmission  work  of  any  kind,  and  when  so  used  are 
mostly  on  special  circuits  of  50  to  150  amperes. 

CASE  II. — Series  motors  worked  at  constant  potential  are 
very  widely  used  for  electric  railway  service  and  other  cases, 
such  as  hoisting,  in  which  great  variations  of  both  speed  and 
torque  are  desirable.  When  supplied  at  constant  potential  the 
speed  of  a  series- wound  motor  varies  widely  with  the  load.  In 
any  case  the  speed  increases  until  the  counter  E.  M.  F.  rises 
high  enough  to  cut  the  current  down  to  the  amount  necessary 
to  give  the  torque  sufficient  for  that  load  and  speed. 

If  the  field  be  strengthened,  the  motor  will  give  a  certain 
output  at  a  lower  speed  than  before;  if  it  be  weakened,  at  a 
higher  speed;  the  torque  being  in  these  cases  correspondingly 
increased  or  decreased. 

The  torque  increases  rapidly  with  the  current,  so  that  when 
the  counter  E.  M.  F.  is  small,  or  zero,  as  in  starting  from  rest, 


90  ELECTRIC   TRANSMISSION  OF  POWER. 

the  torque  is  very  great,  a  property  of  immense  value  in  start- 
ing heavy  loads.  For  in  starting,  not  only  is  the  current 
through  the  armature  large,  but  the  field  is  at  its  maximum 
strength.  If  the  field  strength  varied  directly  as  the  current, 
the  torque  would  vary  nearly  as  the  square  of  the  current. 

As  a  rule,  however,  these,  like  most  other  motors,  are 
worked  with  a  fairly  intense  magnetization  of  the  fields,  so 
that  doubling  the  magnetizing  current  by  no  means  doubles 
the  strength  of  the  field.  In  fact,  most  series  motors  for 
constant  potential  circuits  are  of  the  type  used  for  electric 
railways  and  wound  so  that  the  field  magnets  are  nearly 
saturated  even  with  very  moderate  currents.  Hence  the 
torque  in  such  cases  increases  but  a  trifle  faster  than  the  cur- 
rent. This  construction  is  adopted  in  order  to  reduce  the 
amount  of  iron  necessary  to  secure  a  given  strength  of  field, 
and  so  to  lighten  and  cheapen  the  motor. 

It  is  quite  obvious  that  while  series  motors  at  constant 
potential  have  the  advantage  of  being  able  to  give  on  occasion 
very  great  torque,  they  suffer  from  the  same  disadvantage  as 
constant  current  motors,  in  that  they  are  not  self-regulating 
for  constant  speed.  A  centrifugal  governor  could,  of  course, 
be  arranged  to  do  the  work,  but  since  it  happens  that  most 
work  requiring  great  torque  also  requires  variable  speed, 
nothing  of  the  kind  is  usually  necessary. 

As  previously  explained  the  speed  can  be  easily  regulated  to 
a  certain  extent  by  changing  the  field  strength,  thus  changing 
the  counter  E.  M.  F.,  but  owing  to  the  peculiarity  of  design 
just  noted,  this  method  is  rather  ineffective,  requiring  a  great 
change  in  the  field  winding  for  a  moderate  change  in  speed. 

In  general,  when  a  considerable  range  of  speed  is  needed, 
constant  potential  working  is  abandoned  and  the  speed  is 
changed  by  varying  the  impressed  E.  M.  F.  by  means  of  a 
rheostat.  If  this  E.  M.  F.  be  lowered,  the  current  decreases 
and  the  speed  sags  off  until  the  new  counter  E.  M.  F.  is  low 
enough  to  let  pass  just  enough  current  to  maintain  the  output 
at  the  reduced  speed.  When  the  applied  E.  M.  F.  is  increased 
the  reverse  action  takes  place.  Under  these  circumstances  for 
a  fixed  load  the  current  is  approximately  the  same  independent 
of  the  speed;  for  with  a  uniform  load  the  torque  is  constant, 
while  the  output  (/.  e.  rate  of  driving  the  load)  varies.  Many 


POWER  TRANSMISSION  BY  CONTINUOUS  CURRENTS.         9* 

railway  motors  are  regulated  in  the  manner  just  described, 
although  in  addition  the  field  strength  is  sometimes  varied  by 
cutting  out  or  recombining  fields  and  by  series  parallel  control. 
Rheostatic  control  necessarily  wastes  energy,  and  the  greatest 
recent  improvement  in  railway  practice  consists  in  reducing 
the  E.  M.  F.  applied  to  the  car  motors  by  throwing  the  two 
in  series.  This  secures  a  low  speed  economically  though  the 
rheostat  still  comes  into  play  at  intermediate  speeds. 

Speaking  broadly  then,  series-wound  motors,  while  possess- 
ing many  valuable  properties,  are  limited  in  their  usefulness1 
by.  their  tendency  to  vary  widely  in  speed  when  the  load 


FIG.  33* 

changes.  Hence  they  are  used  chiefly  in  cases  where  the 
speed  is  to  be  varied  deliberately.  A  typical  motor  of  this 
class,  such  as  is  used  for  hoists  and  the  like,  with  rheostatic 
control,  is  shown  in  Fig.  33. 

In  spite  of  the  difficulty  in  regulation,  the  series  motor  pos- 
sesses some  considerable  advantages:  The  field  coils  being  of 
coarse  wire  are  easily  and  cheaply  wound  even  in  motors  for 
very  high  voltage;  the  same  quick  response  to  changes  in 
current  or  load  that  makes  it  hard  to  obtain  uniform  speed  is 
also  most  important  in  many  kinds  of  work;  the  powerful 
initial  torque,  coupled  with  the  kindred  property  of  prompt 
reversal;  all  these  make  the  series  motor  pre-eminent  for  cer- 


92  ELECTRIC   TRANSMISSION   OF  POWER. 

tain  purposes,  especially  where  severe  work  is  to  be  coupled 
with  hard  usage. 

There  is  one  case,  too,  in  which  the  series-wound  motor  can 
be  made  accurately  self-regulating  for  constant  speed — a  case 
somewhat  peculiar  and  unusual,  but  yet  worthy  of  special 
attention. 

CASE  III. — We  have  seen  that  when  the  load  on  a  series 
motor  supplied  at  a  certain  voltage  increases,  the  speed  falls 
off  until  the  increasing  current  due  to  the  lessened  counter 
E.  M.  F.  raises  the  torque  sufficiently  to  meet  the  new 
conditions.  Imagine  now  the  impressed  E.  M.  F.  to  be  so 
varied  that  the  slightest  increase  of  current  in  the  motor 
is  met  by  a  rise  in  the  E,  M.  F.  applied  to  it.  Evidently 
the  speed  would  not  have  to  fall  as  before,  for  the  greater 
applied  voltage  would  furnish  ample  current  for  all  the 
needs  of  the  load.  If  the  variation  in  voltage  could  be  made 
to  depend  on  change  of  torque,  not  giving  the  speed  time  to 
change,  the  regulation  would  be  almost  perfect.  Such  a 
method  has  been  proposed,  but  owing  to  mechanical  difficulties 
has  not  been  used  to  any  extent. 

It  is  possible,  however,  so  to  combine  a  special  motor  and 
generator  that  the  former  will  be  very  closely  uniform  in  speed 
quite  independent  of  the  load.  In  this  connection  we  must 
revert  to  the  properties  of  the  series-wound  dynamo.  If  such 
a  machine  be  driven  at  constant  speed  its  electromotive  force 
will  increase  with  the  current,  since  the  strength  of  field, 
here  the  only  variable  factor  in  the  voltage,  will  increase  with 
the  current.  If  the  field  magnets  of  the  generator  are 
unsaturated,  that  is,  not  so  strongly  magnetized  as  to  require 
considerable  current  to  produce  a  moderate  increase  of  mag- 
netization, they  will  respond  very  promptly  to  an  increase  of 
load  by  raising  the  voltage.  If  such  a  generator  be  connected 
to  a  series-wound  motor  of  proper  design,  the  pair  will  work 
together  almost  as  if  connected  by  a  belt  instead  of  a  long 
line,  and  the  motor  will  run  at  a  nearly  uniform  speed,  since 
the  least  diminution  of  speed,  with  its  accompanying  increase 
of  current,  will  be  met  by  a  rise  in  the  voltage  of  the  genera- 
tor. Such  an  arrangement  is  shown  in  diagram  in  Fig.  34. 

In  this  cut  A  is  the  generator  supplying  current  to  the 
motor  B.  The  machines  should  be  of  practically  the  same 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.        93 

capacity,  for  the  generator  cannot  supply  current  except  to  the 
one  motor  without  disturbing  the  regulation.  Whenever  the 
load  on  B  changes,  a  very  small  reduction  in  speed  suffices  to 
raise  the  voltage  of  A  and  thereby  to  hold  up  the  speed  of  B. 
To  this  end  the  field  magnets  of  B  must  be  more  strongly 
saturated  than  those  of  A,  else  the  same  increase  of  current 
will  raise  the  counter  E.  M.  F.  of  the  motor  and  defeat  the 
purpose  of  the  combination.  If  the  fields  of  the  two  machines, 
are  properly  designed  the  generator  will  increase  its  voltage 
under  increasing  load  just  enough  to  hold  the  motor  at  speed, 
as  a  very  slight  change  in  current  immediately  reacts  on  the 
generator. 


FIG.  34- 

It  is  even  possible  to  make  the  motor  rise  in  speed  under 
load  if  the  generator  is  sufficiently  sensitive  to  changes  of  cur- 
rent. This  is  generally  needless,  but  it  is  often  useful  so  to 
design  A  and  B  that  the  former  will  rise  in  voltage  fast  enough 
not  only  to  compensate  for  the  added  load  on  the  motor  but 
for  the  added  loss  of  energy  in  the  line,  entailed  by  the 
increase  of  current,  thus  regulating  the  motor  even  at  a  long- 
distance. The  difference  of  saturation  between  the  generator 
and  motor  fields  need  not  involve  material  difference  of  design, 
since  it  may  be  effected  by  shunting  the  motor  field.  When 
properly  adjusted  the  system  is  capable  of  holding  the  motor 
speed  constant  within  two  per  cent,  through  the  range  of  load 
for  which  the  machines  are  planned. 


94  ELECl^RIC   TRANSMISSION  OF  POWER. 

It  should  be  noted  in  connection  with  Fig.  34  that,  whereas 
the  current  circulating  in  the  armature  of  a  generator  tends  to 
disturb  the  magnetic  field  in  one  direction,  in  a  motorthe  same 
reaction  is  in  the  opposite  direction.  For  the  current  in  the 
motor  is  driven  through  the  armature  against  the  counter 
E.  M.  F.,  i.  e.  in  the  direction  opposite  to  that  of  the  current 
the  machine  would  give  if  running  as  a  generator.  As  the 
effect  of  the  reaction  is  to  skew  the  direction  of  the  magnetic 
field  that  affects  the  armature  conductors,  and  the  commuta- 
tion must  take  place  when  the  commuted  coil  is  not  under  a 
varying  induction,  the  armature  reaction  compels  one  to  shift 
the  brushes  slightly  away  from  the  position  they  would  have  if 
the  field  were  perfectly  symmetrical.  This  shifting  is  in  the 
direction  of  armature  rotation  in  a  generator,  but  for  the 
reason  above  noted  has  the  opposite  direction  in  a  motor.  In 
either  case  it  should  be  only  a  few  degrees. 

CASE  IV. — Shunt-wound  motors  are  almost  invariably  worked 
on  constant  potential  circuits,  to  which  they  are  particularly 
well  suited.  They  form  by  far  the  largest  class  of  motors  in 
general  use  and  owe  this  advantage  mainly  to  their  beautiful 
self-regulating  properties. 

The  shunt  motor  is  in  construction  practically  the  same  as 
a  shunt-wound  dynamo.  Let  us  look  into  the  action  of  such  a 
machine  when  supplied  from  a  source  of  constant  voltage.  If 
the  design  be  reasonably  efficient,  the  armature  will  have  a 
very  low  resistance  and  the  shunt  circuit,  which  includes  the 
field  coils,  a  resistance  several  hundred  times  greater.  When 
such  a  machine  is  supplied  with  current  of  constant  voltage  at 
its  brushes  and  is  running  at  any  given  speed  and  load,  the 
current  through  the  armature  is  practically  determined  by  the 
counter  E.  M.  F.  developed,  the  armature  resistance  being 
almost  negligible.  The  shunt  is  of  high  resistance  and  takes 
a  certain  small  amount  of  current,  determined  by  the  voltage 
across  the  brushes.  Now  let  the  load  increase;  the  field  is, 
aside  from  loss  of  voltage  on  the  line,  practically  constant,  and 
the  first  effect  of  the  added  load  is,  as  in  a  series  motor,  to 
reduce  the  speed.  But  this  lowers  the  counter  E.  M.  F.,  and 
consequently  the  armature  current  rises  and  the  torque  is 
increased,  thereby  enabling  the  motor  to  operate  under  the 
larger  load.  The  torque  necessary  to  enable  the  motor  to 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.         95 

maintain  an  increased  load  varies  directly  as  the  load  and  is 
also  directly  proportional  to  the  current.  But  since  the  cur- 
rent is  closely  proportional  to  the  difference  between  the 
impressed  and  counter  E.  M.  Fs.,  it  is  possible  to  design  a 
machine  so  as  to  run  at  almost  exactly  constant  speed. 

The  constancy  depends  really  on  the  armature  resistance, 
small  as  it  is.  For  example,  a  motor  is  designed  to  run  at  100 
volts.  Running  light  the  counter  E.  M.  F.  is  99.9  volts  and 
with  an  armature  resistance  of  o.oi  ohm  the  current  will  be  10 
amperes.  The  work  done  is  say  i  HP.  Now  let  a  full  load, 
say  20  HP,  be  thrown  on.  The  torque  will  have  to  be  in- 
creased 20  times,  requiring  200  amperes.  But  this  will  flow 
through  the  armature  under  an  effective  pressure  of  2  volts. 
Hence  the  counter  E.  M.  F.  will  only  have  to  fall  to  98  volts  to 
provide  current  enough  to  meet  the  new  condition.  As  the 
counter  E.  M.  F.  varies  directly  as  the  speed  a  fall  in  speed  of 
less  than  2  per  cent,  will  follow  the  increase  of  load.  This 
computation  neglects  all  questions  of  armature  reaction  as 
well  as  the  effect  of  this  minute  fall  in  speed  on  the  output, 
but  fairly  represents  a  case  that  might  actually  be  met  with  in 
the  best  modern  practice.  In  fact,  shunt  motors  have  been  so 
designed  as  to  vary  no  more  than  i^  per  cent,  in  speed  from 
no  load  to  full  load.  A  variation  of  5  or  6  per  cent,  is,  how- 
ever, more  usual. 

When  supplied  from  an  over  compounded  generator  so  that 
the  impressed  voltage  may  increase  with  the  load,  a  shunt 
motor  can  be  operated  even  more  closely  to  constant  speed 
than  indicated  above,  since  there  is  no  longer  need  for  a  fall 
in  speed  to  maintain  the  requisite  difference  between  the 
impressed  and  counter  E.  M.  Fs.  In  such  case  any  tendency  to 
fall  in  speed  is  at  once  corrected  by  the  rise  in  voltage  on  the 
line.  This  scheme  is  seldom  used,  however,  since  it  is  ill  fitted 
for  simultaneously  operating  a  number  of  motors  at  varying 
loads,  and  for  single  units  has  no  particular  advantages  over 
the  series-wound  pair  previously  noted,  or  a  very  simple 
arrangement  of  alternating  apparatus. 

Not  only  can  the  shunt  motor  be  worked  at  nearly  constant 
speed,  but  it  also  has  the  advantage  of  permitting  a  consid- 
erable range  of  speed  variation  without  sacrificing  much  in  the 
matter  of  efficiency.  We  have  already  .seen  that  a  change  in 


96  ELECTRIC    TRANSMISSION  OF  POWER. 

field  strength  involves  a  change  of  speed,  since  it  necessarily 
alters  the  counter  E.  M.  F. ,  which  in  turn  modifies  the  current. 

In  a  shunt  motor  the  immediate  effect  of  a  decrease  of  field 
strength  is  to  lower  the  counter  E.  M.  F.,  letting  more  current 
through  the  armature  and  increasing  the  torque.  Hence  the 
speed  rises  until  the  current  and  torque  adjust  themselves  to 
the  requirements  of  the  load.  On  the  other  hand,  if  the  field 
be  strengthened,  the  current  necessary  to  carry  the  load  can- 
not be  obtained  without  a  fall  in  speed.  It  is  clear  that  the 
changes  of  speed  thus  obtained  may  be  quite  considerable,  for 
in  a  motor  such  as  that  just  described  a  variation  of  10  per 
cent,  in  the  field  would  produce  an  immense  variation  of  cur- 
rent, which  would  have  to  be  compensated  by  a  change  in 
speed  as  great  as  the  change  in  the  field.  Inasmuch  as  these 
field  changes  can  be  produced  by  varying  the  field  current, 
which  is  always  small,  through  a  rheostat  in  the  circuit,  this 
change  of  field  strength  can  be  accomplished  with  but  a  tri- 
fling waste  of  energy.  If  the  field  magnets  are  comparatively 
unsaturated,  it  is  not  difficult  to  obtain  perhaps  50  per  cent, 
variation  in  speed.  A  motor  designed  for  such  work  is,  how- 
ever, necessarily  bulky,  as  it  must  be  possible  to  get  torque 
enough  to  handle  the  full  load  with  a  field  much  below  its 
normal  strength. 

It  should  be  noted  that  even  when  running  at  a  considerably 
modified  speed,  the  motor  must  still  be  nearly  self-regulating 
for  changes  in  load,  for  the  conditions  that  govern  self-reg- 
ulation are  within  moderate  limits  unaffected  by  the  particular 
strength  of  field  employed.  Only  when  the  armature  reaction 
has  been  greatly  modified  will  the  regulation  be  sensibly 
disturbed. 

A  device  sometimes  used  to  improve  the  regulation  of  motors 
essentially  shunt  wound  is  the  so-called  differential  winding. 
This  consists  of  an  additional  field  winding  in  series  with  the 
armature,  but  around  which  the  current  flows  in  such  a  direction 
as  to  demagnetize  the  field.  The  total  field  strength  is  then 
due  to  the  difference  between  the  magnetizing  power  of  the 
shunt  and  of  this  regulating  coil.  When  the  load  on  the 
motor  increases,  the  additional  current  due  to  a  minute  change 
of  speed  will  weaken  the  field  and  thence  cause  the  motor  to 
run  faster  until  the  counter  E.  M.  F.  adjusts  the  current  to  the 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.        97 

new  speed  and  output.  Differential  winding  obviously  requires 
an  extra  expenditure  of  energy  in  the  field,  since  the  shunt  and 
series  turns  act  against  each  other.  It  is  now  used  much  less 
than  formerly,  since  it  has  been  found  that  a  well-designed 
pure-shunt  motor  will  regulate  very  nearly  as  well  and  more 
efficiently.  Fig.  35  shjws  the  Sprague  motor  wound  on  this 
differential  plan,  now  only  of  historical  interest,  but  which 
through  its  good  qualities  did  much  to  popularize  the  electric 
motor  in  America. 


FIG.  35. 

Various  modifications  of  shunt-  and  series-wound  motors 
have  from  time  to  time  appeared  devised  for  particular  adapta- 
tion to  special  purposes  or  sometimes  merely  for  the  sake  of 
novelty.  None  of  them,  however,  are  of  sufficiently  general 
importance  to  find  a  place  here  except  a  single  very  beautiful 
method  of  obtaining  efficiently  a  very  wide  range  of  speed. 

The  principle-of  this  method  is  to  work  the  motor  at  normal 
full  excitation,  but  to  deliver  to  the  armature  a  current  of 
variable  E.  M.  F.  so  that  a  given  current  and  hence  torque 
may  accompany  very  various  values  of  the  counter  E.  M.  F. 
Fig.  36  shows  the  connections  employed  to  effect  this  result. 
Here  C  is  the  working  motor,  JB  the  special  generator  which 
feeds  its  armature,  A  the  motor  used  to  drive  this  generator, 


98  ELECTRIC    TRANSMISSION  OF  POWER. 

and  D  the  rheostat  and  reversing  switch  in  the  generator  field 
circuit  which  allows  the  generator  E.  M.  F.  to  be  varied.  In 
the  figure  the  motor  H  is  shown  as  a  synchronous  alternating, 
machine  with  a  commutator  from  which  are  fed  the  fields  of 
the  three  machines.  In  ordinary  central  station  practice  A  is 
a  continuous  current  motor  and  the  fields  are  fed  direct  from 
the  distributing  mains.  The  result  of  this  arrangement  is  that 
the  motor  field  C  remains  at  full  strength,  while  the  armature 
current  can  be  brought  to  any  required  strength  at  any  desired 
armature  speed  within  a  very  wide  range.  Hence  C  can  give 
full  load  torque  or  even  more  while  the  armature  is  merely 
turning  over  a  few  times  per  minute,  and  the  speed  can  be 
brought  up  with  the  utmost  delicacy  and  held  at  any  desired 
point.  And  at  every  speed  the  motor  holds  its  speed  fairly 


FIG.  36 

well  irrespective  of  changes  of  load.  For  elevators,  hoists 
and  similar  work  this  device  is  extremely  useful.  The  only 
objection  to  it  is  the  cost  of  installing  the  two  extra  machines, 
which  is  of  course  considerable.  Nevertheless  the  regulation 
attained  is  so  beautiful  and  perfect  that  the  cost  often  becomes 
a  minor  consideration,  and  the  device  is  very  widely  used  in 
cases  where  variable  speed  is  essential. 

POWER    TRANSMISSION    AT    CONSTANT    CURRENT. 

In  its  general  aspect  this  method  must  now  be  regarded  as  a 
makeshift.  It  came  into  existence  at  a  time  when  the  only 
circuits  extensively  installed  were  those  for  arc  lighting,  and 
hence,  if  motors  were  to  be  used  at  all,  they  must  needs  be  of 
the  constant  current  type.  As  incandescent  lighting  became 


POWER  TRANSMISSION  BY  CONTINUOUS   CURRENTS.        99 

more  common  the  arc  motors  were  gradually  replaced  by  shunt 
motors  worked  at  constant  potential.  A  few  constant  current 
plants,  especially  for  motor  service,  have  been  installed  both 
here  and  abroad,  but  for  the  most  part  they  have  merely 
dragged  out  a  precarious  existence,  and  in  this  country  have 
been  abandoned. 

There  is  good  reason  for  this.  The  motors  usually  regulate 
indifferently,  and  there  is  serious  objection  to  running  high 
voltage  wires  into  buildings  when  it  can  be  avoided. 

The  objections  of  the  insurance  companies  alone  are  quite 
sufficient  to  discourage  the  practice.  The  constant  current 
has  often  been  advocated  for  long  distance  transmission  of 
power  where  high  voltage  is  a  necessity.  For  such  service  the 
method  has  the  great  advantage  that  the  motors  do  not  need 
extraordinary  insulation  except  from  the  ground.  A  constant 
potential  service  at  5,000  volts  continuous  current  would  be 
utterly  impracticable,  if  distribution  of  power  in  moderate 
units  were  to  be  attempted,  while  with  constant  currents  it 
is  entirely  feasible,  although  objectionable  on  the  grounds 
mentioned.  In  addition,  unless  a  proposed  transmission  be 
for  power  alone,  the  constant  current  method  shares  with  con- 
stant potential  of  high  voltage  the  very  grave  difficulty  that 
an  incandescent  lamp  service  is  out  of  the  question,  without 
secondary  transformation  of  the  necessarily  high  line  voltage 
to  a  very  moderate  pressure.  This  is  somewhat  expensive  with 
continuous  currents  of  any  kind,  and  at  once  introduces' the 
troublesome  question  of  regulation  at  constant  current  into 
the  problem. 

To  reduce  the  energy  sent  over  a  high  voltage  continuous 
current  line  to  a  pressure  at  which  incandescent  lamps  can  be 
fed,  two  methods  are  possible.  We  may  pass  by  the  plan 
of  using  many  lamps  in  series  as  of  very  limited  appli- 
cability and  forbidden  by  the  fire  underwriters.  First,  the 
required  power  may  be  received  by  a  motor  of  appropriate 
size,  which  is  belted  or  coupled  to  a  low  voltage  generator. 
This  device  does  the  work,  but  it  involves  installing 
three  times  the  capacity  of  the  lamps  desired  in  machinery  of 
a  somewhat  costly  character,  and  losing  in  the  motor  and 
generator  perhaps  15  or  20  per  cent,  of  the  energy  supplied  from 
the  line.  The  other  alternative  is  to  employ  a  composite 


loo  ELECTRIC    TRANSMISSION   OF  POWER. 

machine  combining  the  functions  of  motor  and  generator. 
This  piece  of  apparatus  is  variously  known  as  a  motor  genera- 
tor, dynamotor,  or  continuous  current  converter.  It  is  a 
dynamo  electric  machine  having  a  double-wound  armature  and 
two  commutators.  One  winding  with  its  commutator  receives 
the  high  line  voltage  and  operates  as  a  motor.  The  other 
winding  and  its  commutator  furnishes,  as  a  dynamo,  low 
tension  current.  The  field  is  common  to  both  windings.  Fig. 
37  shows  a  small  machine  of  this  kind,  adapted  to  receive  5,000 
volts  from  the  line,  and  to  deliver  no  volts,  or  vice  versa. 


FIG.  37. 

This  particular  machine  works  at  constant  voltage  on  both 
circuits.  Either  circuit,  however,  could  be  made  to  work  at 
constant  current,  provided  the  means  of  regulation  for  this 
purpose  were  so  chosen  as  to  leave  the  field  and  speed 
unchanged. 

The  cost  of  a  motor  generator,  while  less  than  that  of  two 
separate  machines,  is  still  high,  and  although  its  efficiency  is 
somewhat  greater  than  that  of  the  pair  mentioned  above,  it  is 
obtained  at  the  cost  of  a  rather  complicated  armature,  which, 
from  a  practical  standpoint,  is  quite  objectionable. 


fOWER  TRANSMISSION  BY  CONTINUOUS   CURRENTS.       101 

In  spite  of  the  difficulties  incident  to  working  incandescent 
lamps  from  a  high  voltage  constant  current  circuit,  the  ease 
with  which  such  circuits  can  be  worked,  even  if  for  power 
alone,  at  voltages  far  above  those  available  on  the  constant 
potential  system,  encouraged  their  installation  during  the 
period  between  the  first  efforts  at  long  distance  transmission 
and  the  very  recent  date  at  which  alternating  current  appa- 
ratus has  become  thoroughly  available.  For  some  years  it 
was  constant  current  or  nothing,  so  far  as  long  distance 
transmission,  coupled  with  distribution,  was  concerned. 

As  a  result  of  the  various  adverse  conditions  mentioned, 
transmission  at  constant  current  has  never  made  really  any 
headway  in  American  practice,  and  in  fact  the  method  has  been 
followed  to  a  noticeable  extent  in  only  one  locality — San  Fran- 
cisco. There,  through  the  activity  of  local  exploiters,  constant 
•current  power  circuits  were  early  established  and  remained 
in  fairly  successful  operation  for  several  years. 

There  were  until  recently  three  companies  operating  con- 
stant current  circuits  in  San  Francisco  for  the  distribution 
of  power.  The  currents  employed  were  of  10,  15  and  20  am- 
peres. Most  of  the  motors  were  small,  a  very  large  propor- 
tion of  them  being  under  one  horse  power.  The  total  num- 
ber of  motors  in  circuit  on  the  various  systems  was  between 
•six  and  seven  hundred. 

Except  in  San  Francisco,  what  few  constant  current  motors 
have  been  in  operation  are  nearly  all  on  regular  arc  circuits. 
Their  use  is,  as  a  rule,  discouraged  by  the  operating  companies, 
and  very  few  new  motors  are  being  manufactured  or  sold;  in 
fact,  constant  current  distribution  in  modern  American  practice 
is  almost  non-existent.  Abroad,  the  conditions  are  somewhat 
different,  and  on  the  Continent  constant  current  distribution 
for  long  distance  transmission  work  has  been  exploited  to  a 
very  considerable  extent,  probably  owing  tj  the  early  and 
successful  establishment  of  a  number  of  transmission  plants 
for  single  motors  worked  on  the  series  system.  There  are  sev- 
eral successful  plants  operated  at  constant  current,  one  of  the 
most  considerable  of  them  being  that  at  Genoa,  which  is  an 
excellent  example  of  the  kind  and  as  such  is  worth  more  than 
a  passing  mention,  even  although  the  probability  is  that  it  will 
seldom  be  duplicated,  at  least  on  this  side  the  Atlantic. 


POWER  TRANSMISSION  BY  CONTINUOUS  CURRENTS.      103 

The  Genoa  transmission  is  derived  from  the  River  Gorzente, 
which  about  twenty  years  ago  was  developed  for  hydraulic  pur- 
poses, artificial  lakes  being  established  and  a  tunnel  about  i  ^ 
miles  long  being  built  for  an  outlet.  Beyond  the  tunnel,  an 
aqueduct  some  fifteen  miles  in  length  conveyed  the  water  to 
Genoa,  where  a  considerable  amount  of  power  is  utilized 
directly.  In  this  development  there  was  left  at  the  mouth  of 
the  tunnel  an  unused  fall  of  nearly  1,200  feet  aside  from  the 
head  employed  in  the  aqueduct.  This  has  been  developed 
electrically.  It  was  divided  into  three  partial  falls  of  338,  357 
and  488  feet,  respectively.  At  each  of  these  was  erected  a 
generating  station  with  its  own  transmission  line.  These 
stations  were  named  after  the  three  renowned  electricians, 
Galvani,  Volta  and  Pacinotti.  The  first  mentioned  station 
was  the  first  installed.  It  consists  of  two  generators  operated 
in  series.  Each  is  of  about  50  KW  capacity,  giving  47  amperes 
with  a  maximum  pressure  of  1,100  volts.  Current  is  kept  con- 
stant by  regulating  by  hand  the  speed  of  the  dynamos, 
through  the  gate  which  controls  the  turbines.  Each  dynamo  is 
provided  with  an  automatic  switch,  short  circuiting  the  ma- 
chine in  case  of  extreme  rise  in  voltage.  This  Galvani  station 
was  a  preliminary  or  experimental  station  and  was  followed  up 
by  the  establishment  of  two  others  which  supply  the  power  to 
Genoa.  One  of  these  stations,  which  is  thoroughly  typical  of 
the  system  employed,  is  shown  in  Fig.  38.  It  consists  of  four 
turbines,  each  driving  a  pair  of  dynamos  of  a  little  less  than 
50  KW  output  at  45  amperes  and  about  1,000  volts. 

These  dynamos  are  similar  to  those  in  the  Galvani  station, 
but  the  regulation  for  constant  current  is  obtained  in  a  dif- 
ferent manner.  The  dynamos  are  separately  excited,  the 
fields  being  supplied  in  parallel  from  a  small  dynamo  driven 
by  a  separate  waterwheel.  The  speed  of  this  exciter  is  auto- 
matically varied  by  controlling  its  turbine  in  response  to 
changes  of  current  in  the  circuit.  All  the  dynamos  are  oper- 
ated in  series,  and  like  those  in  the  Galvani  station  are  direct 
coupled  in  pairs.  The  machines  are  insulated  with  enormous 
care,  heavy  layers  of  mica  being  placed  between  the  magnets 
and  the  bed  plates,  while  the  windings  themselves  are  very 
elaborately  protected.  Carbon  brushes  are  employed,  and 
the  commutators  are  reported  to  behave  admirably.  Each 


104  ELECTRIC    TRANSMISSION  OF  POWER. 

• 

dynamo  is  protected  by  most  elaborate  safety  devices,  as  in  the 
Galvani  station.  The  regulation  is  said  to  be  excellent,  even 
under  considerable  changes  of  load. 

The  third  station,  Pacinotti,  contains  eight  machines  of  the 
same  capacity  as  those  in  the  preceding.  They  are  governed 
as  in  the  Galvani  station  by  controlling  their  speed.  This, 
however,  is  done  by  an  electrical  motor-governor  controlled 
by  a  relay  on  the  main  line  and  working  in  one  direction  or 
the  other  as  the  occasion  may  require.  In  all  the  stations  are 
carried  out  the  same  thorough  precautions  regarding  insula- 
tion, and  each  machine  has  around  it  an  insulated  floor  sup- 
ported on  porcelain.  The  line  voltage  from  each  of  the  two 
stations  last  mentioned  is  from  6,000  to  8,000,  and  two 
circuits  are  carried  into  Genoa,  the  extreme  distance  of  trans- 


FIG.  39. 

mission  being  about  eighteen  miles.  The  conductors,  of  wire 
9  mm.  in  diameter,  (nearly  No.  oo  B.  &  S.),  are  for  the  most 
part  bare,  except  when  passing  through  villages,  and  are  sup- 
ported on  oil  insulators  carried  by  wooden  poles,  save  in  some 
few  cases  where  iron  poles  have  been  used.  The  two  circuits 
running  into  Genoa  transmit  power  for  motors  only.  The  loss 
in  the  line  at  full  load  is  8  per  cent. 

At  full  load,  nearly  1,000  HP  is  transmitted  over  the  lines, 
the  motors  being  of  all  sizes  between  10  and  120  horse  power. 
They  are  of  the  ordinary  series-wound  type,  and  their  speed  is 
automatically  controlled  by  centrifugal  governors,  which  act 
by  varying  the  field  strength.  Fig.  39  shows  one  of  the  Genoa 


PO  WER  TRA  N SMI  SSI  ON  B  V  CON  TIN  UO  US  CURRE  N  TS.      105 

motors,  fitted  with  an  automatic  governor  acting  upon  a  corn- 
mutated  field  winding.  They  are  provided  with  carbon 
brushes,  and  are  reported  to  operate  very  successfully. 

It  is  to  be  noted,  however,  that  the  motors  are  placed  in 
special  rooms  with  insulated  floors  and  walls  owing  to  the 
enormous  voltage  which  has  to  be  taken  into  the  buildings. 
They  are  fitted  with  heavy  fly-wheels  to  assist  the  governors, 
and  with  automatic  switches  to  short  circuit  around  the  motor 
in  case  of  excessive  voltage.  The  motors  are  under  the 
special  care  of  skilled  assistants  connected  with  the  staff  of 
the  generating  station,  who  inspect  the  lines  and  go  over  the 
motors  at  intervals  of  a  few  days.  These  extraordinary  pre- 
cautions both  in  the  matter  of  insulation  and  skilled  attendance 
account  in  great  measure  for  the  success  of  what,  under 
American  conditions,  would  have  almost  infallibly  resulted  in 
disastrous  failure.  The  efficiency  of  the  plant  from  turbine 
shaft  to  motor  pulleys  is  said  to  be  a  little  over  70  per  cent. 

As  may  be  judged  from  this  description,  the  whole  instal- 
lation is  of  enormously  complicated  character,  although  per- 
haps as  simple  and  efficient  as  any  alternating  plant  of  the 
same  early  date.  The  plan  of  the  Volta  station  for  the  most 
part  explains  itself.  The  switchboards  for  each  machine  with 
their  plugs  for  connecting  the  pair  of  dynamos  coupled  to  it  are 
shown  at  A,  dynamos  at  B,  the  exciters  at  C,  exciter  switch- 
board and  rheostat  at  D  and  the  solenoids,  which  control  the 
exciter  turbines,  at  E.  Lightning  arresters  are  shown  at  F. 
These  consist  of  a  spark  gap,  impedance  coils  in  series  with 
the  line  and  condenser  shunted  around  them.  Every  motor  is 
provided  with  a  similar  lightning  arrester.  Taken  altogether, 
this  Genoa  plant  is  an  excellent  example  of  the  constant  cur- 
rent system  followed  to  its  legitimate  conclusion.  A  descrip- 
tion of  the  system  is  a  sufficiently  condemnatory  criticism 
judged  from  our  present  point  of  view;  at  the  same  time,  it 
should  be  remembered  that  several  years  ago,  while  this 
station  was  being  built,  the  method  adopted  was  practically  the 
only  one  available  in  the  existing  state  of  the  art,  and  that 
the  system  has  in  more  recent  installations  been  materially 
improved.  Encouraged,  however,  by  the  results  obtained  at 
Genoa,  a  similar  station  was  soon  afterwards  built  delivering  a 
maximum  of  700  HP  at  Brescia  at  an  extreme  full-load  pres- 


io6  ELECTRIC   TRANSMISSION  OF  POWER. 

sure  of  about  15,000  volts  over  a  i2-mile  line.  Since  these 
stations  nearly  a  dozen  others  have  been  installed,  aggregating 
about  17,000  HP,  and  their  performance  has  been  uniformly 
good.  In  spite  of  a  predilection  for  modern  polyphase  work 
one  must  admit  that  a  system  which  has  been  installed  to 
such  an  extent,  and  of  late  in  competition  with  alternating 
methods,  is  far  from  moribund.  Two  strong  points  it 
undoubtedly  has:  freedom  from  all  inductive  disturbances, 
and  the  property  of  carrying  its  extreme  voltage  only  at  full 
load,  the  importance  of  which  will  be  discussed  later.  It  has 
shown  itself  capable  of  doing  steady  and  efficient  work  over 
long  distances  and  under  climatic  conditions  by  no  means 
favorable.  The  Continental  makers  of  this  class  of  machinery 
have  gone  far  beyond  anything  that  has  been  attempted  in 
American  practice  and  have  turned  out  constant  current 
dynamos  of  really  remarkable  properties. 

At  present  machines  of  50  to  60  amperes  have  been  given 
successfully  E.  M.  F.  as  high  as  3,500  volts,  while  those  of 
100  to  150  amperes  have  gone  to  2,500  volts.  As  they  are 
usually  coupled  in  pairs,  a  single  unit  may  have  a  capacity  of 
about  700  KW,  each  component  machine  giving  over  300. 
Without  pushing  beyond  present  apparatus  it  then  becomes 
possible  to  arrange  a  plant  of  1,000  to  1,500  KW  having  a 
working  E.  M.  F.  at  full  load  of  10,000  to  14,000  volts.  Such 
a  plant  is  not  especially  complicated  and  is  quite  as  easy  to 
operate  as  an  alternating  plant.  For  a  load  of  a  few  large 
motors,  it  is  capable  of  thoroughly  good  work,  without,  how- 
ever, presenting  any  striking  advantages  over  a  synchronous 
polyphase  system  save  that  the  line  is  simpler  and  the  motors 
easily  self-starting.  An  alternating  power  station  of  similar 
output  would  contain  practically  as  many  generators,  for  sake 
of  security.  When  it  comes  to  combined  lighting  and  power 
service  the  constant  current  system  is  hard  pushed.  In 
practice  recourse  is  had  to  motor  generators.  Perhaps  the 
best  idea  of  the  situation  may  be  given  by  a  brief  description 
of  the  Swiss  transmission  from  Combe-Garot  to  La  Chaux-de- 
Fonds,  a  distance  of  32  miles.  At  the  former  place  are 
installed  8  generating  units  each  giving  150  amperes  at  1,800 
volts,  giving  a  total  capacity  of  2,160  KW  at  14,400  volts. 
These  generators  are  six  pole  Thury  machines  with  drum 


PLATE   II. 


POWER   TRANSMISSION  BY  CONTINUOUS   CURRENTS.      107 

armatures,  and  are  series  wound.  Regulation  is  by  automatic 
variation  of  the  speed  of  the  turbines,  the  normal  full  load 
speed  being  300  r.  p.  m.  The  line  is  overhead,  of  cables 
having  a  cross  section  of  about  300,000  cm.,  bare  except  in  the 
towns  where  the  power  is  delivered — Locle,  and  La  Chaux-de- 
Fonds  at  the  end  of  the  line.  Motors  aggregating  2,400  HP 
are  in  circuit  at  these  points,  2,000  HP  being  used  in  the 
transforming  stations.  All  motors  above  20  HP  are  upon  the 
high  tension  circuit.  The  substation  at  La  Chaux-de-Fonds 
is  typical  of  the  methods  employed.  It  is  equipped  with 
motor-generators  of  200  HP  working  a  three  wire  system  at 
320  volts  between  the  outside  wires.  One  of  the  motor- 


FIG.  40. 


generator  units  is  shown  in  Plate  II.  It  is  composed  of  two 
six  pole  machines  with  a  fly-wheel  between  them.  The 
machine  to  the  left  is  the  motor  and  upon  it  is  mounted  the 
automatic  speed  regulator.  The  principle  upon  which  this 
works  is  shown  in  Fig.  40.  The  regulating  shunt  around  the 
fields  and  the  brush  shifting  mechanism  are  simultaneously 
actuated  by  the  dogs  thrown  into  gear  by  the  governor.  This 
form  of  governor  is  very  generally  used  for  the  motors  upon 
the  system. 


108  ELECTRIC    TRANSMISSION   OF  POWER. 

The  efficiency  of  both  generator  and  motor  under  test  has 
been  shown  to  be  93.5  per  cent.,  or  87  per  cent,  for  the  com- 
plete unit.  Similar  motor-generators  in  connection  with  a 
storage  battery  furnish  current  at  550  volts  for  railway  service. 

Now  the  drop  in  the  line  at  full  load  is  6  per  cent.,  so  that 
we  are  in  position  to  make  a  very  close  estimate  of  the 
efficiency  of  the  system  from  waterwheel  to  low  tension 
mains.  It  is  obviously 

93-5  X  .94  X  .87  =  76.5  per  cent. 

This  is  a  very  creditable  figure  for  the  total  efficiency,  and  it  is. 
worth  while  comparing  it  with  the  results  ordinarily  reached  in 
polyphase  working.  Taking  the  generator  at  94  per  cent.,  the 
raising  and  reducing  transformers  at  97.5  per  cent,  each,  the 
line  at  .94  and  the  distributing  banks  of  transformers  at  96.5, 
we  have 

.94  x  -94  X  -975  X  .975  X  .965  =  .81. 

The  difference  is  substantially  that  due  to  the  difference  in 
efficiency  between  the  static  transformers  and  the  motor 
generator.  If  the  comparison  be  made  with  the  railway  part 
of  the  proposition,  assuming  the  use  of  a  rotary  converter  the 
case  would  stand  about  as  follows: 

.94  X  -94  X  -975  X  .975  X  -94  =  -79- 

In  the  simple  case  of  large  motors  the  advantage  lies  rather 
the  other  way,  for  the  constant  current  plant  would  show 

•935  X.94  X  .935  =82.1 
against,  for  the  alternating  plant, 

.94  X.94  X  .975  X  .975  X  -94  =.79- 

This  merely  indicates  that  after  passing  the  voltages  which- 
can  be  derived  directly  from  the  armature,  more  is  lost  in  the 
transformers  than  is  gained  in  a  low  voltage  winding.  The 
figures  for  the  efficiency  of  the  alternating  plant  are  taken 
from  actual  data  on  machines  of  about  the  capacity  concerned. 
To  tell  the  unvarnished  truth,  at  voltages  of  15,000  or  more  a. 
constant  current  transmission  coupled  with  a  three  wire  distri- 
bution at  220  to  250  volts  on  a  side  is  capable  of  giving  even 
the  best  alternating  system  a  pretty  hard  rub.  In  this  country- 
no  constant  current  power  transmission  machinery  is  avail- 


POWER   TRANSMISSION  BY  CONTINUOUS   CURRENTS.      109 

able,  but  where  it  is  readily  to  be  had,  it  is  by  no  means  out  of 
the  game.  A  constant  current  transmission  plant  is  now 
under  construction  for  service  between  the  falls  of  the  Rhone 
at  Saint-Maurice,  and  Lausanne,  a  distance  of  34.8  miles. 
The  first  installation  is  of  five  pairs  of  generators  aggregating 
3,300  KW,  giving  at  full  load  a  combined  voltage  of  22,000. 
The  current  in  this  case  is  to  be  150  amperes,  as  in  the 
Combe-Garot  plant  just  described. 

POWER    TRANSMISSION    AT     CONSTANT    POTENTIAL. 

The  transmission  of  power  to  series-wound  motors  at  con- 
stant potential  is  a  branch  of  the  art  which  as  regards  station- 
ary motors  has  been  developed  only  in  special  cases.  It  is, 
however,  the  method  universally  employed  for  electric  rail- 
way work.  One  or  two  sporadic  efforts  have  been  made  to- 
operate  electric  railway  systems  at  constant  current,  but  with 
such  indifferent  success  that  the  method  has  been  abandoned. 
Counting  in  electric  railways,  it  is  safe  to  say  that  at  present 
the  majority  of  all  electrical  power  transmission  in  the  world 
is  done  with  series  motors  worked  at  constant  potential  or  a& 
nearly  constant  potential  as  may  be  practicable.  As  before 
mentioned,  regulation  is  generally  obtained  through  the  use  of 
rheostats  in  series  with  the  motors,  thereby  cutting  down  the 
applied  voltage,  or  by  throwing  the  motors  either  in  parallel 
or  in  series  with  each  other,  or  in  the  third  place  by  a  combi- 
nation of  the  above  methods.  Concerning  the  operation  of 
motors  thus  arranged,  sufficient  has  been  said  to  explain  the 
situation  clearly.  The  general  good  properties  of  the  method 
are  most  prominently  exhibited  in  the  simplicity  of  the  motor 
windings  and  the  very  powerful  effort  which  can  be  obtained 
in  starting  the  motors  from  rest.  These  properties  are  of 
extreme  value  in  railway  service. 

Aside  from  the  operation  of  electric  railways,  series  motors 
at  constant  potential  are  frequently  employed  for  hoists  and 
similar  work  where  a  powerful  starting  torque  and  considerable 
range  of  speed  at  the  will  of  the  operator  are  desirable.  In 
spite  of  the  large  use  of  motors  for  such  purposes  there  are  no- 
plants  either  here  or  abroad  which  may  be  said  to  be  operated 
exclusively  after  this  method,  for  it  is  generally  found  desir- 


110  ELECl^RIC   TRANSMISSION  OF  POWER. 

able  to  combine  in  the  same  system  series-wound  motors  for 
severe  work  and  shunt-wound  motors  for  purposes  where  uni- 
form speed  is  of  prime  importance.  As  a  rule  the  power  trans- 
mission so  accomplished  is  over  a  comparatively  small  distance 
and  really  involves  the  problem  of  distribution  more  than 
transmission  alone.  A  very  large  number  of  electric  hoists 
designed  by  different  makers  are  in  use  at  various  points 
throughout  this  and  other  countries,  doing  service  in  mines, 
operating  elevators  of  one  kind  or  another,  working  derricks  and 
traveling  cranes  and  employed  for  a  large  variety  of  similar 
purposes.  Many  of  the  motors  employed  are  of  the  railway 
type. 

The  voltage  utilized  for  this  work  in  America  at  least  is 
usually  either  200  to  250  volts  or  500  to  600  volts,  the  former 
being  most  generally  used  in  mines,  where  difficulties  of  insu- 
lation are  considerable,  or  in  operating  motors  supplied  by 
three-wire  systems  already  installed.  The  latter  voltage  is  gen- 
erally selected  for  work  above  ground.  None  of  the  plants  so 
equipped  are,  however,  sufficiently  large  or  characteristic  to  be 
worth  a  detailed  description.  The  power  installation  and  the 
method  of  distribution  are  in  general  closely  similar  to  those 
employed  for  electric  railway  work.  Plants  of  higher  volt- 
age than  from  500  to  600  are  so  infrequent  as  to  be  hardly 
worth  considering  in  practical  engineering.  It  is  perfectly 
possible  to  wind  series  motors  for  voltages  considerably 
exceeding  this  figure,  say  for  1,000  or  1,200  volts,  or,  in  rare 
cases,  more,  provided  the  units  are  of  tolerable  size,  but  inas- 
much as  most  plants  for  the  distribution  of  power  require  both 
large  and  small  motors,  wound  both  series  and  shunt,  the 
general  voltage  is  in  nearly  every  case  kept  at  a  point  at  which 
it  will  be  easy  to  meet  these  varied  requirements;  therefore 
500  volts,  the  American  standard  for  railway  practice,  has 
usually  been  selected. 

The  only  noteworthy  exception  may  be  found  in  the  use 
of  the  Edison  three-wire  system  for  distribution  of  power 
to  railway  and  other  motors.  By  this  method  it  becomes 
possible  to  transmit  the  power  at  the  virtual  voltage  of  1,000 
and  to  employ  1,000  volt  motors,  either  series-  or  shunt-wound, 
for  the  larger  units  in  order  to  help  in  preserving  the  general 
balance,  while  at  the  same  time  using  motors  of  all  sizes  with 


POWER   TRANSMISSION  BY  CONTINUOUS   CURRENTS,     m 

any  kind  of  direct  current  winding  connected  between  the 
middle  wire  and  either  of  the  outside  wires.  The  advantages 
of  such  an  arrangement  are  very  evident,  and  if  the  number 
of  motors  be  considerable,  so  that  it  is  possible  to  balance  the 
system  with  a  fair  degree  of  accuracy,  we  have  at  our  disposal 
a  very  convenient  method  for  the  distribution  of  continuous 
currents.  It  is  interesting  to  note  that  this  scheme  found  its 
first  considerable  development  in  electric  railway  service  itself. 
Of  course  the  use  of  both  no  and  220  volt  motors  on  Edison 
three-wire  systems  is  very  common,  but  the  extension  of  the 
plan  to  operating  electric  roads,  and  under  conditions  which 
as  regards  balance  are  somewhat  trying,  is  a  considerable 
step  toward  an  individual  method. 

The  method  of  working  electric  railways  on  the  three-wire 
system  is  well  shown  in  Fig.  41.     Here'  the  road  is  a  double 


FIG.  41. 

track  one,  to  which  the  method  is  generally  best  suited. 
The  ground^  track  and  supplementary  wires  serve  as  a  neutral 
wire,  both  tracks  being  placed  in  parallel  for  this  purpose 
and  thoroughly  bonded.  On  a  double  track  road,  the  cars 
running  on  each  side  of  the  system  will  be  substantially  the 
same  in  number,  and  if  the  total  number  of  cars  be  consider- 
able, a  very  fair  balance  can  be  obtained,  although  never  as 
good  as  is  customarily  and  necessarily  used  in  an  Edison  three- 
wire  system  for  lighting.  In  order  to  still  further  improve 
the  balance  of  the  system  and  prevent  its  being  disturbed  as 
might  otherwise  occur  by  a  blockade  on  one  track  at  some 
point,  it  is  better  to  make  the  trolley  wire  above  each  track 
consist  of  sections  of  alternate  polarity  and  of  convenient 
length,  so  that  even  in  case  of  a  blockade,  stopping  a 


112  ELECTRIC   TRANSMISSION  OF  POWER. 

considerable  number  of  cars,  the  load  would  be  removed 
almost  equally  from  both  sides  of  the  system. 

Installed  in  this  way,  a  railway  system  is  operated  at  a  virtual 
voltage  of  1,000,  and  the  saving  of  copper  over  the  ordinary 
distribution  at  500  volts  is  considerable  in  spite  of  the  inevitable 
lack  of  balance,  and  the  loss  of  the  track  as  part  of  the  main 
conducting  system. 

The  three-wire  method  is  not  confined  to  double  track 
roads,  as  even  where  only  a  single  track  is  employed  various 
sections  or  branches  of  it  may  be  linked  together  to  form  a 
three-wire  system.  It  is  interesting  to  note  that  this  plan  has 
been  in  use  in  several  American  street  railway  plants  during 
the  past  few  years,  notably  at  Bangor,  Me.,  and  at  Portland, 
Ore.  Apart  from  such  railway  service,  the  three-wire  distri- 
bution has  been  very  little  used  for  power  transmission  pur- 
poses either  here  or  abroad,  but  no  particular  difficulties  are 
involved  and  it  occasionally  may  prove  a  very  serviceable 
method  for  the  distribution  of  power  over  moderate  distances. 

INTERDEPENDENT    DYNAMOS    AND    MOTORS. 

Aside  from  the  distribution  of  power  for  railway  purposes, 
by  far  the  most  interesting  kind  of  power  transmission  by 
continuous  currents  is  that  in  which  a  special  combination  of 
two  series  machines  is  employed,  giving  a  self-regulating 
system  comprising  a  motor  unit  and  a  generator  unit.  This 
plan  has  been  widely  and  successfully  used  abroad,  but  has 
not  been  employed  in  American  engineering  practice  except 
in  an  experimental  and  tentative  way,  owing  largely  to  the 
difficulties  that  have  been  encountered  in  the  production  of 
large  direct-current  generators  for  high  voltage. 

While  it  is  not  at  alt  a  difficult  matter  to  build  a  machine 
giving  five  or  six  thousand  volts  with  a  rather  small  cur- 
rent, such  as  is  used  in  arc  lighting,  the  troubles  at  the  com- 
mutator have  proved  forbidding  when  any  attempt  has  been 
made  to  use  currents  large  enough  to  obtain  units  of  any 
considerable  size.  Tne  whole  subject  of  power  transmission 
has  been  but  recently  taken  up  seriously  in  this  country,  all 
the  energy  of  the  electrical  art  being  concentrated  in  lighting 
and  railway  work,  and  so  far  as  the  development  has  now 
taken  place,  it  has  been  almost  wholly  in  the  'ine  of  alternat- 


POWER  TRANSMISSION  BY  CONTINUOUS   CURRENTS,      nj 

ing  current  work.  It  is  quite  obvious  that  a  system  of  power 
transmission  such  as  we  are  considering  possesses  very  great 
convenience  where  single  units  are  to  be  operated  over  rela- 
tively long  distances.  In  the  first  place,  the  inductive  difficul- 
ties familiar  with  alternating  currents  are  avoided.  In  the 
second  place,  the  motors  are  self-starting  under  load,  a  condi- 
tion that  has  not  been  true  of  alternating  machinery  until  the 
introduction  of  the  polyphase  system.  Through  the  energy 
of  several  foreign  engineers,  notably  Mr.  C.  E.  L.  Brown, 
much  was  done  in  power  transmission  by  this  method  long 
before  alternating  current  apparatus  had  been  suitably  devel- 
oped. The  same  difficulties  were  encountered  abroad  as 
here.  It  proved  to  be  very  difficult  to  build  machines  of  suffi- 
cient voltage  and  of  any  considerable  output. 

In  this  connection  it  is  noteworthy  that  nearly  all  the  plants 
of  this  character  on  the  Continent  have  been  installed  at  rela- 
tively low  voltages,  most  of  them  less  tha.n  1,000,  correspond- 
ing in  general  character  to  the  American  plants  over  similar 
distances  worked  at  constant  potential.  In  the  very  few 
instances  where  long  distances  have  been  attempted,  the 
usual  method  has  been  to  employ  generators  and  motors  per- 
manently connected  together  in  series,  on  account  of  the 
impracticability  of  getting  sufficient  power  in  one  unit  at  a 
very  high  voltage.  This  proceeding  somewhat  complicates 
the  system.  In  addition,  the  generators  and  motors  have  to 
be  especially  designed  for  each  ether  in  order  to  secure  regu- 
lation; which,  of  itself,  is  a  considerable  disadvantage. 

This  last  difficulty  may  be  in  part  avoided  by  using  a  shunt 
around  the  field  coils  of  the  generator,  thereby  changing  its 
regulation  under  variations  of  current.  A  similar  device  is 
widely  used  in  this  country  in  connection  with  compound- 
wound  generators,  where  a  shunt  applied  across  the  terminals 
of  the  series  coils  is  used  to  regulate  the  compounding.  In 
either  case,  the  obvious  result  of  such  a  shunt  is  to  diminish 
the  change  in  the  field  produced  by  a  given  increase  in  cur- 
rent. In  this  way  the  necessity  for  special  machines  can  be 
partially  obviated.  The  plants  installed  on  this  peculiar  series 
plan  have  been  uniformly  successful,  and  permit  of  the  con- 
venient transmission  of  moderate  amounts  of  power  over  con- 
siderable distances.  Such  plants  have  even  been  employed 


H4  ELECTRIC    TRANSMISSION   OF  POWER. 

in  connection  with  motor-generators  to  supply  a  general 
distribution  system,  though  evidently  at  a  high  cost  for 
apparatus. 

In  order  to  distribute  low  tension  currents  from  such  a 
transmission  system,  it  is  necessary  to  employ  either  a  motor, 
coupled  to  a  dynamo,  or  a  composite  machine  with  a  double 
winding,  combining  both  functions.  Either  alternative  in- 
volves the  loss  of  energy  substantially  equivalent  to  that 
lost  in  two  dynamo-electric  machines  of  the  capacities  con- 
cerned. These  losses  are  necessarily  much  more  serious  than 
those  in  an  alternating  current  transformer.  They  are  li'kely 
to  amount  to  from  12  to  15  per  cent.,  so  that  quite  aside  from 
the  efficiency  of  the  generating  dynamo  and  of  the  line,  the 
price  paid  for  the  privilege  of  obtaining  a  low  tension  current 
is  considerable. 

For  the  delivery  of  power  alone,  where  motors  in  series 
coupled  to  appropriate  generators  can  be  used,  the  method 
is  well  fitted  for  use  under  certain  circumstances  and  is 
closely  approximate  in  efficiency  to  that  which  would  be 
obtained  by  an  alternating  current  transmission  over  the  same 
distance.  It  is  interesting  to  know  that  one  of  the  longest  lines 
over  which  electrical  power  to  any  considerable  amount  is 
transmitted  is  operated  on  this  series  direct  current  system. 
Many  longer  alternating  transmissions  are  under  construc- 
tion, but  only  a  few  are  yet  in  operation. 

The  plant  in  question  is  that  which  is  used  in  operating  the 
Biberest  Paper  Mills,  near  Soluere,  Switzerland.  The  power 
is  derived  from  the  River  Suze,  near  Bienne,  and  the  distance 
of  transmission  is  a  little  less  than  twenty  miles.  At  the 
generating  station  the  available  head  of  water  is  about  forty- 
five  feet,  and  the  quantity  is  sufficient  to  generate  about  400  HP. 
The  power  station  contains  a  400-HP  turbine  running  at  120 
revs,  per  minute,  of  which  the  vertical  shaft  is  connected  by 
means  of  beveled  gear  to  two  I30-KW  dynamos.  They  are  six 
pole  machines,  Gramme  wound,  and  give  at  275  revs,  per 
minute  about  40  amperes  at  3,300  volts.  The  two  machines 
are  connected  in  series,  giving  a  working  potential  of  6,600 
volts  on  the  line.  It  should  be  noted  that  great  care  is  taken 
in  insulating  them,  the  bed  plates  being  carried  on  porcelain 
insulators.  Carbon  brushes  are  employed.  The  line  is  a  bare 


POWER    TRANSMISSION  BY  CONTINUOUS   CURRENTS.     115 

copper  wire,  7  millimetres  in  diameter,  about  No.  i  B.  &  S. 
gauge.  The  line  runs  through  a  mountainous  country,  and  is 
liberally  provided  with  lightning  arresters  at  various  points. 

The  two  motors  at  the  mills  are  duplicates  of  the  gener- 
ators, the  only  modifications  being  those  to  insure  their  self- 
regulation.  They  run  at  200  revs,  per  minute  on  6,000  volts 
delivered,  and  give  about  155  HP  each.  The  commercial 
efficiency  of  this  interesting  system  is  somewhat  in  excess  of 
75  per  cent,  at  full  load.  Fig.  42  shows  one  of  the  motors  on 


FIG.  42. 

its  foundation,  and  its  coupling  to  its  mate.  This  plant  is  the 
most  striking  example  of  long  distance  transmission  by  series- 
wound  interdependent  generators  and  motors,  and  probably 
exhibits  the  system  at  its  best. 

In  this  country  the  above  system  has  not  been  used  in 
anything  more  than  an  experimental  way,  owing  principally 
to  two  reasons:  first,  for  moderate  distances,  involving  not 
more  than  1,000  to  1,500  volts,  shunt-wound  generators  and 
motors  working  on  either  the  two-wire  or  three-wire  systems 
afford  better  opportunity  for  distribution,  inasmuch  as  their 
use  is  not  limited  to  single  mechanical  units;  second,  no  seri- 
ous demand  for  long  distance  transmission  arose  in  America 


Il6  ELECTRIC   TRANSMISSION  OF  POWER. 

prior  to  the  development  of  the  alternating  system  to  the  point 
at  which  alternating  motors  became  thoroughly  practicable.  It 
has  been  characteristic  of  American  electrical  engineering  that 
it  has  occupied  itself  with  one  thing  at  a  time.  The  develop- 
ment of  the  electric  light  was  followed  by  a  concentration  of 
energy  on  the  electric  railroad,  and  this  has  only  recently  been 
succeeded  by  extensive  power  transmission  enterprises,  often 
in  themselves  involving  railway  work.  Such  a  mental  habit 
is  not  conducive  to  an  even  development,  but  probably  accom- 
plishes quite  as  much  as  a  more  symmetrical  advance. 

CONSTANT    POTENTIAL    SYSTEMS. 

Shunt-  or  compound-wound  generators  used  in  connection 
with  shunt-wound  motors  have  found  very  extensive  use  in 
this  country  in  transmissions  over  moderate  distances.  The 
very  obvious  advantage  of  such  a  system  is  that  it  permits  the 
ready  distribution  of  power  as  well  as  its  easy  transmission. 
If  it  becomes  necessary  to  transmit  power  from  one  point  to 
another,  the  chances  are  much  more  than  even  that  at  the 
distributing  end  of  the  line  it  will  be  desirable  to  utilize  the 
power  in  a  number  of  units  of  varying  size.  Such  an  arrange- 
ment bars  out  transmission  from  series  dynamos  unless  upon 
the  constant  current  system  with  its  inherent  difficulties  of 
regulation,  whereas  with  shunt-wound  apparatus  the  problem 
is  easy.  It  often  happens,  as  previously  mentioned,  that  at 
the  receiving  end  both  series  and  shunt  motors  are  used,  the 
former  for  hoisting  and  similar  work,  the  latter  for  operation 
at  constant  speed. 

The  growth  of  the  electric  railwayhas  encouraged  the  estab- 
lishment of  such  transmission  plants,  and  their  number  is  very 
considerable,  scattered  over  all  parts  of  the  Union,  not  a  few 
of  them  being  in  the  mining  regions  of  the  Rocky  Mountains 
and  on  the  Pacific  coast,  as  well  as  in  various  isolated  plants 
through  the  rest  of  the  country.  In  most  of  them,  the  dis- 
tances being  moderate,  an  initial  voltage  of  from  500  to  600 
has  been  employed;  more  rarely,  voltages  ranging  from  1,000 
to  i, 800.  Such  plants  have  been  uniformly  successful  and 
have  done  sterling  service  for  some  years  past.  The  efficiency 
of  this  method  of  transmission  is  about  the  same  as  that  of  the 


POWER    TRANSMISSION  BY  CONTINUOUS  CURRENTS.      117 

series  method,  just  described,  but  with  the  advantage  that  the 
shunt  motor  supplied  at  constant  potential  can  advantageously 
be  distributed  wherever  the  work  is  to  be  done,  while  with  inter- 
dependent series  units  any  distribution  of  power  has  to  be  ac- 
complished by  means  of  shafting  and  belting  or  its  equivalent. 

The  net  efficiency  from  generator  to  driven  machine  is  likely 
to  be  rather  better  with  the  transmission  at  constant  potential 
than  in  the  case  just  discussed.  The  generators  and  motors 
are  of  nearly  the  same  efficiency;  the  line  at  ordinary  dis- 
tances is  customarily  worked  at  about  the  same  pressure  in 
both  methods,  but  distribution  by  shafting  is  far  less  efficient 
at  any  but  short  distances  than  distribution  of  electric  power 
by  wire.  The  loss  from  the  centre  of  distribution  to  individual 
motors  will  very  seldom  exceed  5  per  cent.,  while  the  loss  in 
equivalent  shafting  will  seldom  be  less  than  10  per  cent.,  and 
more  often  15  or  more;  in  fact,  it  generally  turns  out  upon 
investigation  that  so  far  as  efficiency  is  concerned  there  is 
a  noticeable  saving  in  transmitting  power  electrically,  even 
within  the  limits  of  a  mill  or  large  factory,  over  the  results 
which  can  be  obtained  by  the  use  of  transmission  by  shafts 
and  belts.  In  a  large  building  where  the  power  is  to  be  widely 
distributed,  it  seldom  happens  that  the  loss  in  the  shafting  is 
less  than  25  per  cent.  Anything  in  excess  of  this  figure  repre- 
sents remarkably  good  practice.  With  motors,  80  per  cent, 
efficiency,  if  the  units  are  of  tolerable  size,  can  be  reached 
without  much  difficulty,  and  there  are  comparatively  few  cases 
where  the  efficiency  need  fall  lower  than  75.  In  such  a  plant, 
recently  installed  in  a  Belgian  gun  factory,  and  described  in 
the  last  chapter,  the  guaranteed  efficiency  was  76.6  per  cent. 
As  the  efficiency  of  the  dynamo  was  reckoned  at  but  90  per 
cent.,  the  total  efficiency  would  in  practice  be  raised  without 
difficulty  to  78  or  79  per  cent,  at  full  load. 

As  regards  efficiency  in  general,  aside  from  the  disadvan- 
tages previously  mentioned  in  changing  the  voltage  of  direct 
current  circuits,  the  efficiency  of  transmission  by  such  currents 
is  in  itself  as  high  as  has  ever  been  reached  by  other  means. 
There  is  no  material  difference  between  the  efficiency  of  direct 
and  alternating  current  generators,  nor  between  the  efficiency 
of  direct  current  motors  and  the  polyphase  motors,  at  least, 
among  alternating  motors.  In  these  particulars,  the  direct 


n  ELECTRIC    TRANSMISSION   OF  POWER. 

current  is  able  to  hold  its  own  against  all  comers  and  in  the 
cost  of  motors  it  has  at  present  a  material  advantage. 

As  regards  transmission  of  power  over  considerable  dis- 
tances, a  case  has  already  been  mentioned  in  which  the  result 
is  as  good  as  can  reasonably  be  expected.  Direct  current, 
however,  continually  runs  into  the  limitation  of  available  volt- 
age as  soon  as  distribution  is  to  be  attempted.  Where  single 
or  a  few  large  motor  units  are  to  be  used,  consisting  of  either 
single  machines  or  groups  operated  as  a  unit,  the  efficiency  of 
the  system  is  likely  to  be  as  high  as  that  obtained  from  units. 
of  similar  magnitude  on  alternating  current  systems.  The 
only  disadvantage  of  the  direct  current  in  point  of  efficiency 
in  this  particular  case  is  that  if  the  amount  of  power  to  be 
transmitted  be  large,  it  is  necessary  to  use  generators  and 
motors  coupled  in  series,  while  if  alternating  currents  were 
used,  one  would  have  the  advantage  of  employing  a  single 
machine  of  equivalent  capacity.  The  principal  disadvantage 
of  direct  current  machinery  is  the  commutator,  which  at  high 
voltages  is  likely  to  be  sooner  or  later  the  source  of  consider- 
able trouble.  Careful  mechanical  and  electrical  construction 
may  materially  reduce  this  difficulty,  but  it  always  remains  to. 
be  faced,  and  is  liable  at  any  time  to  become  troublesome. 

On  long  lines,  the  direct  current  has  the  advantage  of  pro- 
ducing no  inductance  in  the  line,  an  advantage,  however, 
which  does  not  apply  to  plants  which  can  advantageously  be 
operated  as  single  units.  Such  a  single  unit  system,  arranged 
for  alternating  currents,  can  have  the  inductance  of  the  cir- 
cuit completely  nullified  by  the  simple  expedient  of  strength- 
ening the  field  of  the  motor. 

It  must  be  remembered,  however,  that  in  several  particulars 
continuous  current  has  peculiar  advantages.  In  the  first 
place,  it  is  well  known  that  a  direct  current  is  decidedly  less, 
dangerous  than  an  alternating  current  of  the  same  nominal 
voltage,  so  far  as  the  question  of  life  is  concerned.  The 
difference  between  the  two  is  even  greater  than  would  be  indi- 
cated by  the  difference  in  maximum  voltage. 

An  alternating  current  has  a  maximum  voltage  of  approxi- 
mately 1.4  times  its  mean  effective  voltage,  and  in  addition  to 
this  an  alternating  current  is  certainly  intrinsically  more 
dangerous  by  reason  of  the  greater  shock  to  the  nervous 


POWER    TRANSMISSION  BY  CONTINUOUS  CURRENTS.      119 

system  produced  by  the  alternations  of  E.  M.  F.  The  ease 
of  transforming  alternating  current  to  a  lower  voltage  partially 
obviates  this  objection,  but  the  fact  remains.  So  far  as 
danger  of  fire  is  concerned,  the  continuous  current  has  the 
power  of  maintaining  a  much  more  formidable  arc  than  an 
alternating  current  of  the  same  effective  voltage;  but,  on  the 
other  hand,  the  alternating  current  has  somewhat  greater 
maximum  voltage  with  which  to  start  the  arc,  so  that,  practi- 
cally, honors  are  even. 

The  increase  of  experience  with  resonance  and  kindred 
phenomena,  acquired  on  long  lines  and  at  high  voltages,  has 
emphasized  the  fact  that  alternating  transmission  work  is  not 
exactly  a  bed  of  roses  for  the  engineer,  and  when  it  comes  to 
a  question  of  transmission  at  50,000  volts  or  so,  difficulties 
multiply.  At  and  above  this  pressure,  there  can  be  little 
doubt  that  insulation  is  a  very  difficult  task,  and  there  is 
equally  little  doubt  that  of  two  lines,  one  constant  current 
and  the  other  polyphase,  transmitting  the  same  energy  at  the 
same  effective  voltage,  the  former  would  be  in  trouble  much 
less  frequently  than  the  latter.  In  the  first  case  there  are  but 
two  wires  involved,  while  in  the  second  there  are  certain  to  be 
three,  and  generally  considerations  of  inductance  would  lead 
to  not  less  than  six  in  a  plant  of  large  size.  And  when  the 
point  is  reached  where  insulation  is  a  costly  matter  the  extra 
wires  and  precautions  are  likely  to  outweigh  any  intrinsic 
saving  in  copper.  The  constant  current  plant,  too,  always  has 
the  advantage  that  it  is  only  working  at  its  maximum  voltage 
during  the  peak  of  the  load  and  the  rest  of  the  time  has  a  very 
considerable  factor  of  safety. 

Whether  the  increased  cost  and  complication  of  the  gener- 
ating station  of  a  constant  current  system  can  be  endured  for 
the  sake  of  these  advantages  is  a  matter  open  to  discussion; 
it  certainly  cannot  be  answered  in  the  negative  offhand,  how- 
ever. The  continuity  of  service  possible  in  an  alternating 
plant  at  50,000  volts  and  above  is  an  unknown  quantity,  and 
in  the  absence  of  data  upon  this  point  one  is  not  justified  in 
estimating  the  importance  of  an  alternative  method. 

It  is  a  mistake,  however,  to  suppose  that  the  considerably 
increased  maximum  voltage  in  an  alternating  current  involves 
much  greater  danger  of  leakage,  or  of  breaking  down  insula- 


120  ELECTRIC    TRANSMISSION   OF  POWER. 

tion  under  all  circumstances.  Under  many  conditions  it  is 
highly  probable  that  the  electrolytic  strain  from  continuous 
current  on  insulating  materials,  particularly  when  damp,  is 
more  destructive  than  the  added  electrostatic  strain  of  an 
alternating  current.  Within  any  voltages  now  regularly 
employed,  the  total  difference  is  probably  immaterial.  In  the 
matter  of  one  of  the  great  dangers  to  an  overhead  line  and 
apparatus,  *'.  e.,  injury  from  lightning,  direct  current  has  a  very 
material  advantage  in  that  it  is  possible  to  use  coils  of  con- 
siderable self-induction  in  connection  with  such  circuits,  so 
as  to  keep  oscillatory  discharges,  like  lightning,  out  of  the 
machines.  This  is  well  shown  in  the  singular  freedom  of  arc 
lighting  stations  from  serious  damages  to  the  machines  by 
lightning,  as  compared  with  stations  containing  other  kinds  of 
electrical  apparatus.  In  this  case  the  magnets  of  the  arc 
machines  themselves  act  as  a  powerful  inductance,  tending  to 
throw  the  lightning  to  earth.  High  voltage,  shunt-wound 
dynamos  and  alternators  are  much  more  sensitive  in  this 
respect. 

Consequently,  part  of  the  price  one  has  to  pay  for  the  privi- 
lege of  utilizing  alternating  currents  is  extra  care  with  respect 
to  protective  devices  against  lightning.  In  the  present  state 
of  the  art,  the  best  field  for  combined  transmission  and  dis- 
tribution of  power  by  continuous  currents  is  in  cases  involving 
distribution  over  moderate  distances,  within,  say,  a  couple  of 
miles  from  the  centre  of  distribution,  and  even  then  in  prob- 
lems where  lighting  is  not  an  essential  part  of  the  work.  The 
voltage  of  a  lighting  circuit  is  determined  by  the  voltage  of 
the  Tamps  which  can  be  employed  upon  it,  and  this  is  so 
limited  that  if  lighting  is  to  be  done  on  the  same  circuit 
as  power  distribution  there  are  few  cases  where  such  a  com- 
bined system  can  be  successfully  used  with  continuous  cur- 
rents. The  field  seems  at  present  to  be  somewhat  widened 
by  the  advent  of  3-wire  systems  at  220  to  250  volts  on  a  side, 
but  their  place  in  the  art  is  hardly  yet  secure,  although  their 
use  is  extending. 

At  all  long  distances  continuous  current  is  at  a  disadvantage 
in  point  of  available  voltage  where  distribution  is  to  be  done, 
and  has  in  most  cases  no  very  material  advantages  for  single 
unit  work.  It  will  require  considerable  further  advance  in 


POWER   TRANSMISSION  BY  CONTINUOUS  CURRENTS.      121 

dynamo  building  to  render  continuous  current  thoroughly 
available  for  high  voltages,  and  even  then  only  in  units  of 
moderate  size,  say  300  to  400  KW.  In  this  lies  its  weakness. 
Its  strength  is  largely  in  its  present  firm  foothold  in  electrical 
practice,  and  in  the  fact  that  standard  apparatus  of  this  kind 
is  available  everywhere  and  is  manufactured  cheaply  in  large 
quantities  by  numerous  makers.  It  is,  furthermore,  inter- 
changeable to  a  degree  which  will  never  be  true  of  alternating- 
current  machinery  until  there  is  far  greater  unity  in  alter- 
nating-current practice  than  we  are  likely  to  have  for  some 
years  to  come. 


CHAPTER   IV. 

SOME    PROPERTIES    OF    ALTERNATING    CIRCUITS. 

WE  have  already  seen  in  Chapter  I  that  the  current  nor- 
mally produced  by  a  dynamo-electric  machine  is  an  alternating 
one,  so  that  a  continuous  current  exists  in  the  external  circuit 
only  in  virtue  of  the  commutator.  Until  within  the  last 
decade  the  original  alternating  current  was  utilized  to  but  a 
trivial  extent.  Nevertheless  it  possesses  certain  properties 
so  valuable  that  their  practical  development  has  wrought  a 
revolution  in  applied  electricity. 

To  describe  these  properties  with  any  degree  of  complete- 
ness would  require  several  volumes  the  size  of  the  present,  and 
would  involve  mathematical  considerations  so  abstruse  as  to  be 
absolutely  unintelligible  to  any  save  the  professional  reader. 
We  shall  therefore  at  the  very  start  drop  the  academic  methods 
of  treatment  and  confine  ourselves,  so  far  as  possible,  to  the 
physical  facts  concerning  those  properties  of  alternating  cur- 
rents which  have  a  direct  bearing  on  the  electrical  transmission 
of  energy.  This  discussion  will  therefore  be  somewhat  uncon- 
ventional in  form,  although  adhering  rigidly  to  the  results  of 
experiment  and  mathematical  theory.  The  student  who  is 
interested  in  the  exact  development  of  this  theory  will  do 
well  to  consult  the  excellent  treatises  of  Fleming,  Mascart 
and  Joubert,  Bedell  and  Crehore,  and  Steinmetz,  all  of  which 
are  full  of  valuable  demonstrations. 

The  fundamental  differences  between  the  behavior  of  con- 
tinuous and  of  alternating  currents  lie  in  the  fact  that  in  the 
former  case  we  deal  mainly  with  the  phenomena  of  a  flow  of 
electrical  energy  already  steadily  established,  while  in  the 
latter  case  the  phenomena  of  starting  and  stopping  this  flow  are 
of  primary  importance.  These  differences  are  akin  to  those 
which  exist  between  keeping  a  railway  train  in  steady  motion 
over  a  uniform  track,  and  bringing  it  up  to  speed  from  a 
state  of  rest.  In  steady  running  the  amount  of,  and  the 

122 


SOME   PROPERTIES  OF  ALTERNATING   CIRCUITS.      123 

variations  in  the  power  needed,  depend  almost  wholly  on  the 
friction  of  the  various  parts,  while  in  starting  both  the  power 
and  its  variations  are  profoundly  affected  by  the  inertia  of  the 
mass,  the  elasticity  of  the  parts,  and  other  things  that  cut 
little  figure  when  the  train  is  up  to  a  uniform  speed. 

The  characteristic  properties  of  alternating  currents  are  due 
mainly  to  the  starting  and  stopping  conditions,  and  are  only  in- 
cidentally affected  by  the  circumstance  that  the  flow  of  current 
alternates  in  direction.  As,  however,  this  alternating  type  of 
current  is  in  general  use  and  its  uniform  oscillations  give  the 
best  possible  opportunity  for  observing  the  effect  of  repeated 
stops  and  starts,  we  will  look  into  the  generation  of  alternat- 
ing current,  not  forgetting  that  for  certain  purposes  we  shall 


FIG.  43. 

have  to  recur  to  the  phenomena  of  a  single  stop  or  start  in  the 
current. 

Fig.  43  shows  an  idealized  generator  of  alternating  currents. 
It  is  composed  of  a  single  loop  of  wire  arranged  to  turn  con- 
tinuously in  the  space  between  the  poles  of  a  magnet.  This 
space  is  a  region  of  intense  electromagnetic  stress  directed 
from  pole  to  pole,  as  indicated  by  the  dotted  lines.  The  two 
€nds  of  the  loop  are  connected  to  two  insulated  metallic  rings 
connected  by  brushes  to  the  terminals  A  and  B  of  the  external 
circuit.  We  have  already  seen  that  what  we  call  electromotive 
force  appears  whenever  the  electromagnetic  stress  about  a 
conductor  changes  in  magnitude.  Now  in  turning  the  loop  as 
shown  by  the  arrow,  the  electromagnetic  stress  through  it 
changes,  and  of  course  sets  up  an  electromotive  force.  In 
the  initial  position  of  the  loop  shown  in  Fig.  43.  it  includes 
evidently  the  maximum  area  under  stress;  after  it  has  turned 
through  an  angle  a,  this  area  will  be  much  lessened,  and  when 
a  =  90°,  the  loop  will  be  parallel  to  the  plane  of  the  electro- 
magnetic stress  and  hence  can  include  none  of  it  at  all. 


124 


ELECTRIC    TRANSMISSION   OF  POWER. 


But  the  resulting  electromotive  stress  is,  other  things  being 
equal,  proportional  to  the  rate  at  which  work  is  expended  in 
uniformly  turning  the  coil;  /.  e.  it  is  proportional  to  the  rate 
of  change  in  the  electromagnetic  stress  included  by  the  coil. 
This  rate  is,  during  a  single  revolution,  greatest  when  the 
sides  of  the  loop  are  moving  directly  across  the  lines  of  stress, 
and  least  when  moving  nearly  parallel  to  them.  Hence  we  see 
from  Fig.  43  that  the  electromotive  force  in  our  coil  will  be  a 
maximum  when  a  =  90°  or  270°  and  a  minimum  in  the  two 
intermediate  positions.  For  a  simple  loop  it  is  easy  to  compute 
exactly  the  way  in  which  the  electromotive  force  will  vary  as 
the  loop  turns.  The  area  of  strain  included  by  the  coil  in  any 


FIG.  44. 

position  is  proportional  to  the  cosine  of  the  angle  a,  hence  for 
uniform  motion  the  rate  of  change  in  the  area  is  proportional 
to  the  sine  of  a.  Therefore  the  E.  M.  F.  at  every  point  of  the 
revolution  is  proportional  to  sine  a. 

If  now  we  draw  a  horizontal  line  and  measure  along  it  equal 
distances  corresponding  to  degrees,  and  then,  erecting  at  each 
degree  a  line  in  length  proportional  to  the  sine  of  that  par- 
ticular angle,  join  the  ends  of  these  perpendiculars,  we  shall 
have  an  exact  picture  of  the  way  in  which  the  E.  M.  F.  of 
our  loop  rises  and  falls.  Fig.  44  is  such  a  curve  of  E.  M.  F. — 
a  so-called  "sine  wave,"  which  is  expressed  by  the  equation, 
E  =  E^  sin  at. 

This  simple  form  of  E.  M.  F.  curve — the  "sine  wave" — is 
assumed  to  exist  in  most  mathematical  discussions  of  alternat- 
ing current  to  avoid  the  frightful  complications  which  would 
result  from  assuming  such  E.  M.  F.  curves  as  often  are  found  in 
practice.  This  assumption  is  somewhat  rash,  for  a  true  sine 
wave  is  never  given  by  any  practical  generator,  but  the  error 
does  not  often  invalidate  any  of  the  conclusions,  for  the  exact 


SOME  PROPERTIES  OF  ALTERNATING   CIRCUITS.      125 

form  of  the  wave  only  matters  in  discussing  certain  cases,  where 
it  can  often  be  taken  into  account  without  much  difficulty. 

Actual  alternating  generators  give  curves  of  E.  M.  F.  greatly 
influenced  by  the  existence  of  an  iron  armature  core  which 
collects  the  lines  of  force  so  that  as  the  core  turns  the  change 
of  stress  through  the  armature  coils  is  not  directly  propor- 
tional to  anything  in  particular.  A  glance  at  the  rudimentary 
dynamo  of  Fig.  8,  Chapter  I,  will  suggest  the  reason.  It  is  evi- 
dent enough  that  the  armature  could  turn  almost  30°  from  the 
horizontal  with  scarcely  any  change  in  the  magnetic  relations 


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depressed  top,  since  the  rate  of  change  of  induction  would  be 
very  moderate  when  it  should  be  considerable.  The  practical 
bearings  of  wave  form  on  power  transmission  work  will  be  taken 
up  in  the  next  chapter.  At  present  it  will  suffice  to  say  that  the 
best  standard  alternators  give  a  fairly  close  approximation  to 
the  sine  form.  Fig.  45  shows  the  E.  M.  F.  curve  of  one  of  the 
great  Niagara  generators.  This  is  an  excellent  example  of 
modern  practice  and  shows  a  form  slightly  flatter  than  the  sine 
curve  and  with  a  mere  trace  of  depression  at  the  crest.  Plenty 
of  machines  are  in  operation,  however,  that  give  curves  not 
within  hailing  distance  of  being  sinusoidal — e.  g.  Fig.  46,  which 
shows  the  E.  M.  F.  curve  of  one  of  the  earliest  alternators 
designed  for  electric  welding.  In  this  case  there  is  a  far  sharper 
wave  than  the  sinusoidal,  of  a  curious  toothed  form.  Many  of 
the  early  alternators  with  ironclad  armatures  gave  curves  quite 
far  from  the  sine  form,  generally  rather  pointed,  while  the 
tendency  in  recent  machines  has  been  rather  in  the  opposite 


126 


ELECTRIC   TRANSMISSION   OF  POWER. 


direction,  toward  curves  like  Fig.  45,  although  seldom  so  nearly 
sinusoidal.     The  general  equation  for  their  E.  M.  F.  is, 
£=j5l  sin  at  -\-£3  sin  3  at  -}-£&  sin  5  at . . .  .^^sin  (2n  —  i)  at. 
In  other  words  the  E.  M.   F.  is  built  up  of  the  fundamental 
frequency  and  its  odd  harmonics. 

Now  as  to  the  current  produced  by  this  oscillating  electro- 
motive force.     In  ordinary  work  with  continuous  currents,  the 


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FIG.  46. 

current  corresponding  to  each  successive  value  of  the  E.  M.  F. 

would  be  very  easjr  to  determine  by  simple  reference  to  Ohm's 
27- 

law,  C  =  — .     If  the  dynamo  of  Fig.  43  gave  one  volt  maximum 
./i 

E.  M.  F.  and  were  connected  through  a  simple  circuit  of  one 
ohm  resistance  the  maximum  current  would  be  one  ampere, 
and  the  current  at  all  points  would  be  directly  proportional  to 
the  voltage.  Hence  if  the  E.  M.  F.  varied  as  shown  in  Fig.  44, 
the  current  would  vary  in  precisely  the  same  manner  and  the 
curve  showing  its  variation  would,  if  drawn  to  the  same  scale, 
exactly  coincide  with  the  curve  of  Fig.  44.  This  would  be 
generally  true  if  we  had  only  resistance  to  consider,  and  the 
treatment  of  alternating  currents  would  then  be  very  simple. 


SOME  PROPERTIES  OF  ALTERNATING  CIRCUITS.      127 

But  the  starting  and  stopping  of  current  which  takes  place 
periodically  in  alternating  circuits  produces  great  changes  in 
the  electromagnetic  stresses  about  the  conductors,  and  these 
changes  are  in  turn  capable  of  very  important  reactions. 
They  give  to  the  alternating  current  its  most  valuable  proper- 
ties, but  also  involve  its  action  in  very  curious  complications. 

Turn  back  to  Chapter  I  and  examine  Fig.  4.  We  see  from 
it  that  whenever  the  electromagnetic  stresses  about  a  circuit 
as  A,  change  by  the  variation  of  the  current  flowing  in  it,  an 
E.  M.  F.  is  set  up  in  the  parallel  circuit  B,  opposing  the 
change  of  E.  M.  F.  in  A.  This  fact,  as  we  shall  see  later,  is 
the  root  of  the  alternating-current  transformer.  Suppose  now 
that  in  the  circuit  of  our  alternator  is  a  coil  of  wire  wound  in 


FIG.  47. 

close  loops  as  shown  in  Fig.  47.  Here  A  and  B  are  the 
dynamo  terminals,  C  the  general  circuit  and  D  the  aforesaid 
coil.  Let  an  E.  M.  F.  be  started  in  the  direction  A  C  D. 
The  resulting  current  flows  through  Z>,  but  the  electromag- 
netic stresses  set  up  by  the  current  about  for  instance,  the 
loop  ^,  produce  an  E.  M.  F.  in  neighboring  coils  tending  to 
drive  current  in  a  direction  opposite  to  that  from  the  dynamo. 
In  other  words  e  acts  toward  f  just  as  A  acted  toward  B  in 
Fig.  4.  Thus  each  turn  tends  to  oppose  the  increase  of  cur- 
rent in  the  others.  When  the  current  in  C  ceases  to  vary,  of 
course  the  reactive  E.  M.  Fs.  in  e  and  /  stop,  for  there  is 
for  the  moment  no  change  of  stress  to  produce  them,  but  as 
the  main  current  begins  to  decrease,  the  reactive  E.  M.  Fs. 
set  in  again. 

The  main  or  "impressed"  E.  M.  F.  is  thus  opposed  in  all 
its  changes  by  the  reactive  or  "inductive"  E.  M.  F.  due  to 
the  combined  action  of  the  loops  at  Z>.  Hence  the  impressed 
E.  M.  F.  in  driving  current  through  the  system  has  to  over- 
come, not  only  the  resistance  of  the  conductors,  but  opposing 
electromotive  forces.  Therefore  since  a  part  of  the  impressed 


*2S  ELECTRIC    TRANSMISSION  OF  POWER. 

E.  M.  F.  is  taken  up  with  neutralizing  the  inductive  E.  M.  F., 
only  the  remainder  is  effective  against  the  true  resistance  of 
the  circuit.  Ohm's  law,  then,  cannot  apply  to  alternating  cir- 
cuits in  which  there  is  inductive  action,  except  in  so  far  as  we 
deal  with  the  "  effective  "  E.  M.  F.  The  relation  between  the 

F,  E 

impressed  E.  M.  F.  and  the  current  is  not  C-— ,  but  C=  — 

less  a  quantity  depending  on  the  amount  of  inductive  E.  M.  F. 
encountered. 

This  state  of  things  leads  to  two  very  important  results: 
First,  the  current  in  an  inductive  circuit  is  less  than  the 
impressed  E.  M.  F.  would  indicate.  Second,  this  current 


FIG.  48. 

reaches  its  maximum  later  than  the  impressed  E.  M.  F.  For 
the  current  depends  on  the  effective  E.  M.  F.,  and  for  each 
particular  value  of  this  the  impressed  E.  M.  F.  must  have  had 
time  to  rise  enough  to  overcome  the  corresponding  value  of 
the  inductive  E.  M.  F.  The  current  is  thus  damped  in  amount 
and  caused  to  lag  in  "phase"  as  shown  in  Fig.  48.  The 
heavy  line  here  shows  the  variations  of  the  impressed  E.  M.  F. 
and  the  light  line  the  corresponding  variations  of  current  in  a 
circuit  containing  inductive  reaction — inductance.  The  dis- 
tance a  b  represents  the  "angle  of  lag,"  while  b  c  is  180°  as 
shown  in  Fig.  44.  Very  similar  relations  are  found  in  prac- 
tice, although  the  lag  is  often  greater  than  shown  in  the 
cut,  particularly  when  alternating  motors  are  in  circuit.  Fig. 
49  shows  the  curves  of  E.  M.  F.  and  current  from  a  very  small 
alternating  motor  at  the  moment  of  starting.  The  angle  of 
lag  in  this  case  is  a  trifle  over  45°,  and  the  curves  are  much 
closer  to  sine  waves  than  is  usual. 

The  inductive  E.  M.  F.,  as  has  already  been  explained,  is  due 


SOME  PROPERTIES  OF  ALTERNATING  CIRCUITS.      129 

to  the  magnetic  changes  produced  by  the  variation  of  the  cur- 
rent. Just  as  in  the  dynamo  of  Fig.  43,  the  actual  amount  of 
E.  M.  F.  is  directly  proportional  to  the  rate  of  change  in  mag- 
netic stress,  which  is  in  turn  proportional  to  the  change  of  cur- 
rent. The  inductive  E.  M.  F.  is  therefore  at  every  point 
proportional  to  the  rate  of  variation  of  the  current.  But  the 
current  wave  is,  like  the  impressed  E.  M.  F.  wave,  still  approxi- 
mately a  sine  curve,  for  it  has  been  merely  shifted  back 
through  the  angle  of  lag  and  although  damped  it  has  been 


180a 


FIG.  49. 


simply  changed  to  a  different  scale.  Being  still  essentially 
a  sine  curve  its  rate  of  variation  is  a  cosine  curve,  or  what 
is  the  same  thing  a  sine  curve  shifted  backward  a  quarter 
period,  90°.  Indeed  this  is  at  once  evident,  for  since  the 
current  varies  most  slowly  at  its  maximum,  the  inductive 
E.  M.  F.  must  be  a  minimum  at  that  point,  i.  e.  it  must  be 
90°  behind  the  current  in  phase,  while  since  E.  M.  F.  and  cur- 
rent vary  symmetrically,  in  general  the  forms  of  the  two  curves 
will  be  similar.  The  effective  E.  M.  F.  which  is  actually 
engaged  in  driving  the  current  is  a  wave  in  phase  with  the 
current  it  drives  and  of  similar  shape,  /.  e.  a  sine  curve. 

We  have  then  in  an  inductive  circuit  three  E.  M.  Fs.  to  be 
considered : 

I.  The  impressed  E.  M.  F.,  acting  on  the  circuit. 

II.  The  inductive  E.  M.  F.,  opposing  I. 

III.  The  effective  E.  M.  F.,  the  resultant  of  I.  and  II. 
Plotting  the  respective  curves,  they  bear  to  each  other  the 

relation  shown  in  Fig.  50.  Here  a  is  the  impressed  E.  M.  F., 
b  the  effective  E.  M.  F.  (or  the  current)  lagging  behind  a 


I3o 


ELECTRIC   TRANSMISSION  OF  POWER. 


through  an  angle  usually  denoted  by  cp,  and  c  is  the  induc- 
tive E.  M.  F.  90°  behind  b.  Now  since  b  is  the  resultant  of 
the  interaction  of  a  and  <r,  and  we  know  that  b  and  c  are  90° 
apart  in  phase,  it  is  comparatively  easy  to  find  the  exact  rela- 
tion between  the  three. 

For  we  can  treat  electromotive  forces  acting  at  known 
angles  with  each  other  just  as  we  would  treat  any  other 
forces  working  conjointly.  If  for  example  we  have  a  force 
A  B,  Fig.  51,  acting  simultaneously  with  a  force  B  C,  at  right 
angles  to  it,  the  magnitudes  of  the  forces  being  proportional 
to  the  lengths  of  the  lines,  the  result  is  the  same  as  if 
a  single  force  in  magnitude  and  direction  A  C  were  working 


FIG.  50. 


FIG.  51. 


instead  of  the  two  components.     This  is  a  familiar  general 
theorem  that  proves  particularly  useful  in  the  case  in  hand. 

If  we  take  A  B  equal  to  the  effective  E.  M.  F.  and  B  C  equal 
to  the  inductive  E.  M.  F.,  then  A  C  is  the  impressed  E.  M.  F. 
It  at  once  appears  that  the  angle  between  A  B  and  A  C  is  cp, 
the  angle  of  lag.  Then  from  elementary  trigonometry  it 
appears  that 

A  B  =  A  C  cos  cp 

C  B  =  A  C  s\n  cp  =  A  B  tan  cpt  hence 
CB 


We  are  therefore  in  a  position  to  determine  the  three 
E.  M.  Fs.  and  the  angle  cp,  knowing  any  two  of  the  four 
quantities.  Thus  for  a  given  impressed  E.  M.  F.,  E,  such  as  is 
found  on  any  constant-potential  alternating  circuit,  the  effec- 
tive E.  M.  F.  which  determines  the  current  is  given  by  E  cos 
cp.  As  cp  grows  less  and  less  through  decrease  of  the  induc- 
tive E.  M.  F,,  A  C,  the  impressed  E.  M.  F.  necessary  for  a 
given  current,  also  decreases,  and  finally  when  cp  becomes 


SOME  PROPERTIES  OF  ALTERNATING   CIRCUITS.      I31 

zero,  A  C  =  A  B.  In  other  words  the  impressed  E.  M.  F.  is 
then  simply  that  needed  to  overcome  the  ohmic  resistance. 

For  any  particular  current,  then,  A  B  is  directly  propor- 
tional to  the  resistance  of  the  circuit,  while  C  B  is  directly 
proportional  to  the  "  inductance  "  of  the  circuit,  that  prop- 
erty of  the  particular  circuit  which  determines  the  inductive 
E.  M.  F.  Calling  this  /  we  may  redraw  Fig.  51  in  a  very 
convenient  form  —  Fig.  52.  Here  we  see  the  relation  between 
R  and  I  in  determining  the  impressed  E.  M.  F.  necessary  to 
drive  a  certain  current  through  an  inductive  circuit.  The 
magnitude  of  the  E.  M.  F.  evidently  is  *JR*  4~  I*  if  the  units 
of  measurement  are  chosen  correctly,  and  it  is  always  pro- 
portional to  this  quantity,  which  is  related  to  the  impressed 
E.  M.  F.  as  resistance  is  to  the  effective  E.  M.  F.  j 

Hence  y'  R1  -|-  F  has  sometimes  been  called  "apparent 
resistance."  The  more  general  name,  however,  is  impedance, 
which  indicates  the  perfectly  general  relation  between  E.  M.  F. 
and  current.  If  /be  zero,  as  in  a  continuous-current  circuit, 
then  the  impedance  becomes  the  simple  resistance.  We  can 
now  write  out  some  of  the  general  relations  of  current  and 
E.  M.  F.  in  alternating  circuits  as  follows,  calling  E  the  im- 
pressed E.  M.  F.  as  before: 


E  =  C  ^/R1  -}-  / 
and  with  respect  to  the  angle  of  lag, 


/  =  R  tan  cp 


tan  g,  =  - 


Hence  knowing  the  angle  of  lag  and  the  resistance  of  a 
circuit  the  inductance  can  be  found  at  once.  The  angle 
of  lag,  depending  on  the  ratio  of  /  and  R,  must  be  the  same 
for  all  circuits  in  which  this  ratio  is  the  same.  Also  in 


132  ELECTRIC   TRANSMISSION-  OF  POWER. 

any  circuit  of  given  inductance  increasing  the  resistance 
diminishes  the  angle  of  lag,  while  of  course  also  diminishing 
the  current  for  a  given  value  of  E.  In  fact  since  /  does  not 
represent  work  done,  for  the  inductive  E.  M.  F.  represents 
merely  a  certain  amount  subtracted  from  the  impressed 
E.  M.  F.  by  the  reaction  of  the  circuit,  any  process  which  for 
a  given  value  of  E  increases  the  energy  actually  spent  in  the 
circuit  is  accompanied  by  a  diminution  of  the  angle  of  lag. 

This  freedom  of  the  circuit  from  any  energy  losses  due  to 
/  is  a  fact  of  the  greatest  importance.  It  is  fully  borne  out 
by  experiment  and  there  is  besides  good  physical  reason  for 
it.  For  since  current  and  E.  M.  F.  are  the  two  factors  of 
electrical  energy  there  can  be  no  energy  when  the  product  of 


R 
FIG.  52.  FIG.  53. 

these  factors  is  zero.  Note  now  Fig.  53,  developed  from  Fig. 
50.  Hence  a  is  the  line  of  zero  E.  M.  F.  and  current,  b  the 
current  curve  for  a  single  alternation,  and  c  the  correspond- 
ing curve  of  inductive  E.  M.  F.  90°  behind  the  current. 
When  b  is  a  maximum  c  is  zero  and  vice  versa.  And  since 
c  is  equally  above  and  below  the  zero  line  during  each  alterna- 
tion of  current,  the  average  inductive  E.  M.  F.  is  zero  and 
the  average  energy  throughout  the  alternation  is  zero.  The 
same  conditions  would  evidently  continue  if  instead  of  an 
alternation  we  took  a  complete  cycle  (i.  e.  the  whole  curve 
from  the  time  the  current  starts  in  a  given  direction  until 
it  starts  in  the  same  direction  again)  or  any  number  of 
cycles.  Thus  /  must  be  entirely  dropped  out  of  considera- 
tion in  discussing  the  question  of  work  done  in  an  alternating 
circuit.  And  since  E  differs  from  E^  the  effective  E.  M.  F., 
only  by  a  function  of  /,  the  energy  value  of  which  is  zero,  the 
energy  in  the  circuit  is  exactly  measured  by  Ev  and  the  corre- 


SOME  PROPERTIES  OF  ALTERNATING  CIRCUITS.      133 

spending  current  which,  as  we  have  seen,  is  in  phase  with  it. 
But 

E^  =  E  cos  (p. 

Hence  multiplying  both  members  of  this  equation  by  C  to 
reduce  to  energy, 

Energy  =  C  Ev  =.  C  E  cos  (p. 

That  is,  the  energy  in  an  alternating  circuit  is  equal  not  to 
the  impressed  E.  M.  F.  multiplied  by  the  current  but  to  their 
product  multiplied  by  the  cosine  of  the  angle  of  lag.  The 
product  C  E  is  sometimes  called  the  apparent  energy  to  dis- 
tinguish it  from  C  E^  the  actual  energy.  This  apparent 
energy  is  that  obtained  by  measuring  the  amperes  and  the 
impressed  volts  and  taking  their  product.  The  real  energy 
is  that  which  would  be  obtained  by  putting  a  wattmeter  in 
•circuit.  Hence 

watts 


—  r—  — 

volt-amperes 


=  cos 


a  convenient  and  common  method  of  measuring  the  angle  of 
lag.  If  in  addition  the  value  of  /is  wanted  it  can  be  obtained 
at  once  from  the  expression  for  tangent  cp  already  given. 

We  thus  see  that  the  energy  in  an  inductive  circuit  is  not 
directly  proportional  to  the  voltage  as  measured  but  to  the 
effective  voltage,  which  is  less  by  an  amount  depending  on 
the  inductance.  This  difference  is  sometimes  referred  to  as 
the  "  inductive  drop  "  in  a  circuit.  The  result  is  that  to  drive 
a  given  current  through  an  inductive  circuit  the  generator 
must  give  a  voltage  depending  on  the  impedance  of  the  cir- 
cuit. On  the  other  hand  if  an  inductive  circuit  be  fed  from 
a  given  impressed  E.  M.  F.  the  current  required  to  represent 
a  given  amount  of  energy  exceeds  that  required  in  a  non-induc- 
tive circuit  in  the  ratio  of  i  to  the  cosine  of  the  angle  of  lag. 
The  net  result,  then,  of  inductance  in  an  alternating  circuit  is 
to  increase  the  E.  M.  F.  at  the  generator  required  to  produce 
a  given  E.  M.  F.  at  the  load,  and  to  increase  the  current 
required  to  deliver  a  given  amount  of  energy. 

The  E.  M.  F.  and  current  are  here  supposed  to  be  meas- 
ured in  the  ordinary  way,  by  properly  designed  voltmeters 


134  ELECTRIC   TRANSMISSION  OF  POWER. 

and  ammeters.  In  power  transmission  work  inductance  in  the 
circuit  (line  or  load  or  both)  means  that  the  dynamo  has  to 
give  voltage  enough  to  overcome  the  impedance  of  the  system 
and  still  to  deliver  the  proper  number  of  volts  at  the  motor, 
while  the  motor  will  take  extra  current  enough  to  compensate 
for  the  lag  between  the  E.  M.  F.  at  its  terminals  and  the 
resulting  current. 

The  dynamo  thus  has  to  be  capable  of  giving  a  little  extra 
voltage  and  the  motor  must  be  able  to  stand  a  little  extra  cur- 
rent. In  other  words  both  machines  must  have  sufficient  mar- 
gin in  capacity  to  take  care  of  this  matter  of  lagging  current. 

We  have  already  seen  the  general  relation  between  resist- 
ance, inductance,  and  impedance.  Let  us  now  look  into  the 
quantity  last  mentioned  so  as  to  see  its  numerical  relation  to 
the  others.  If  a  circuit  has  a  certain  resistance  in  ohms  and 
a  given  inductance,  what  is  its  impedance,  /.  e.  the  ratio 
between  the  measured  voltage  and  the  measured  current? 

The  real  question  involved  is  the  value  of  the  inductive  E. 
M.  F.  This,  like  any  other  E.  M.  F.,  is  proportional  to  the 
rate  of  variation  of  the  electromagnetic  stress  which  produces 
it.  Its  total  magnitude  depends  on  the  rate  of  variation  of  the 
current  and  the  ability  of  this  current  to  set  up  stresses  which 
can  affect  neighboring  conductors  as  in  Fig.  43.  This  latter 
property  depends  on  the  number  of  turns,  their  locality  with 
reference  to  each  other,  and  other  similar  conditions  which 
depend  simply  on  the  physical  nature  of  the  circuit  and  so  for 
any  given  circuit  are  settled  once  for  all.  These  properties 
are  denned  on  the  basis  of  their  net  effect,  and  the  ratio  of  the 
rate  of  variation  of  the  current  to  the  inductive  E.  M.  F.  pro- 
duced by  it  in  a  given  circuit  is  usually  known  as  Z,  the 
" coefficient  of  self  induction"  of  that  circuit.  The  total 
inductive  E.  M.  F.  is  then  equal  to  Z,  multiplied  by  the  actual 
rate  of  current  variation  expressed  in  such  units  as  will  fit  the 
general  system  by  which  E,  R  and  other  quantities  are  con- 
cordantly  measured. 

Expressed  in  this  way  the  rate  of  current  variation  in  an 
alternating  circuit  is  2  n  n,  where  n  is  the  number  of  cycles  per 
second,  and  n  has  its  ordinary  meaning  of  3.1416.  Hence  the 
inductance  of  the  circuit  is  numerically  2  n  n  L,  the  last  factor 
being  dependent  on  the  nature  of  the  circuit  and  denoting 


SOME   PROPERTIES   OF  ALTERNATING   CIRCUITS.      135 

the  inductance  per  unit  rate  of  current  variation.  The  2  n  n 
factor  gives  the  actual  rate  of  current  variation,  which  may 
change  to  any  amount,  while  L  remains  fixed.  L  therefore 
may  at  all  times  in  a  given  circuit  be  expressed  in  terms  of 
any  unit  that  is  conveniently  related  to  other  electrical  units. 

Such  a  unit  inductance  is  the  henry,  which  is  the  inductance 
corresponding  to  an  inductive  E.  M.  F.  of  i  volt  when  the 
inducing  current  varies  at  the  rate  of  i  ampere  per  second. 

If  therefore  L  for  any  circuit  is  known  in  henrys  the  total 
inductance  /is  6.28  n  L. 

We  are  now  ready  to  apply  a  numerical  value  to  /  in  Fig. 
52  and  the  resulting  equations. 

For  example  let  us  suppose  that  a  certain  alternating  circuit 
has  a  resistance  of  100  ohms  and  L  =  o.  i  henry.  The  im- 

C 


B 
FIG.  54. 

pressed  E.  M.  F.  is  1,000  volts.  What  will  be  the  current  and 
its  angle  of  lag  ?  Lay  off  A  B  Fig.  54,  100  units  long.  Then 
at  B,  to  the  same  scale  erect  a  perpendicular  B  C,  2  it  n  Lin. 
height.  If  we  are  dealing  with  an  alternating  circuit  of  6o~ 
per  second,  such  as  is  often  used  for  power  transmission,  2 
TC  n  L  will  be  37.7  units  high.  Now  join  A  C  and  the  result- 
ing length  on  the  same  scale  is  the  impedance  in  ohms.  But 


AC---  ^IF  +  F  =  /v/ioo2+  37.  f  =  106.9  nearly.  And  since 
the  current  equals  the  impressed  E.  M.  F.  divided  by  the 
impedance,  the  current  in  this  case  would  be  9.36  amperes 
instead  of  the  10  amperes  due  if  there  had  been  no  inductance. 

And  since  tan  cp  =  —  it  is  here  .377,  which  corresponds  to  an 
J\. 

angle  of  20°  40'. 

Also,  since  E  =  C  ^/R*  -f-  /2,  we  can  readily  find  the  im- 
pressed E.  M.  F.  required  to  produce  in  this  circuit  any  given 
current.  For  C—  10  amperes,  ^"  =  1,069  volts  and  so  on. 


r 

We  have  seen  that  cos  <p  =  —  :  -    so   that   in    the 

volt-amperes 


136 


ELECTRIC   TRANSMISSION  OF  POWER. 


case  in  hand  where  cos  cp  =  .936  the  actual  energy  in  the  cir- 
cuit is  93,6  per  cent,  of  that  indicated  by  the  readings  of  volt- 
meter and  ammeter. 

This  factor,  cos  <p,  connecting  the  apparent  and  the  real 
energy,  is  known  as  the  " power  factor"  of  the  circuit. 

As  /  —  R  tan  ^?,  and  in  any  given  case  n  is  known,  L  can 
readily  be  obtained  from  a  measurement  of  lag  in  a  circuit  of 
known  resistance.  It  must  be  remembered,  however,  that  if 
the  inductance  is  due  to  a  coil  having  an  iron  core,  the  value 
of  L  will  change  when  the  magnetization  of  the  iron  changes, 
so  that  results  obtained  with  a  certain  current  will  not  hold 
exactly  for  other  currents.  The  values  of  L  found  in  practice 
cover  a  very  wide  range  from  a  few  thousandths  of  a  henry  in 


B-2J& 

FIG.  55. 

a  small  bit  of  apparatus  like  an  electric  bell,  to  some  hundreds 
of  henrys  in  the  field  magnets  of  a  big  dynamo.  L  in  fact  is 
nearly  as  variable  as  R. 

As  a  practical  example  in  inductance  effects  we  may  consider 
the  effect  of  alternating  current  in  a  long  straightaway  circuit. 
Suppose  for  example  we  have  a  circuit  50,000  ft.  long  composed 
of  No.  4  B.  &  S.  copper  wires.  The  wires  are  about  i  ft.  apart 
and  about  20  ft.  above  the  ground.  What  voltage  will  be 
required  to  deliver  10  amperes  through  this  circuit  at  130 
cycles  per  second,  and  what  will  be  the  angle  of  lag?  The 
resistance  of  this  wire  is  0.25  ohms  per  1,000  ft.  Z,  its 
coefficient  of  induction,  is  .0003  henry  per  1,000  ft.  The 
total  resistance  of  the  circuit  is  then  25  ohms  and  its  total 
inductance,  /,  =  6.28  X  130  X  .03  =  24.5.  Plotting  as  before 
these  values,  in  Fig.  55  we  have  the  impedance  equal  to 


SOME   PROPERTIES  OF  ALTERNATING   CIRCUITS.      137 


/Y/252  -}-  24.  5a  =  35  ohms.  Hence  E  must  be  350  volts  instead 
of  the  250  that  would  suffice  in  the  case  of  continuous  current. 

Tan  cp  =  -—    =  .98.     The    corresponding   angle  is   44°    25'. 

The  ratio  of  impedance  to  resistance  in  this  case  is  1.4:  i.  This 
ratio,  often  called  the  impedance  factor,  is  a  very  convenient 
way-  of  treating  the  matter,  and  tables  giving  its  value  for 
common  cases  will  be  given  later.  In  case  of  apparatus  being 
connected  to  the  circuit,  the  computation  of  its  effect  is  easy. 
If  it  has  resistance  Rl  and  inductance  71  then  the  total  impe- 
dance of  the  circuit  will  be  ^/  (R  -f  Jtiy  +  (/  +  /')'  and 
so  on  for  any  number  of  resistances  and  inductances,  the 
impedance  being  always  equal  to  the  square  root  of  the 
squared  sum  of  the  resistances  plus  the  squared  sum  of  the 
the  inductances.  Thus  an  inductance  added  anywhere  in 
circuit  changes  the  total  impedance  and  the  angle  of  lag. 

There  are  several  ways  of  looking  at  inductance,  according 
as  one  wishes  to  deal  more  particularly  with  inductive  E.M.F., 
the  changes  in  electromagnetic  stress  which  produce  it,  or 
the  energy  changes  which  accompany  it.  The  first  point  of 
view  is  the  one  here  taken,  in  accordance  with  the  definition 
of  the  henry  just  given.  Hence  the  henry  may  be  called  unit 
inductance,  in  which  case  the  quantity  /which  we  have  been 
considering  measures  the  inductive  E.  M.  F.,  and  since  it  is 
the  product  of  the  inductance  for  unit  rate  of  current  change 
multiplied  by  2  TT  n,  it  is  sometimes  referred  to  as  inductance- 
speed^  now  conventionally  termed  reactance. 

In  alternating-current  working  inductance  may  easily  be- 
come quite  troublesome,  through  the  ''inductive  drop"  in 
the  line  and  the  necessity  of  sometimes  delivering  a  current 
quite  out  of  proportion  to  the  energy.  Thus  in  alternating- 
current  lighting  plants  during  the  hours  of  daylight  when  the 
actual  load  is  small,  the  current  may  be  of  quite  imposing  size 
from  the  lag  produced  by  the  inductance  of  the  unloaded 
transformers  in  circuit.  The  sort  of  thing  which  happens  may 
readily  be  figured  out.  Suppose  we  are  dealing  with  a  trans- 
former or  other  inductive  apparatus  having  a  resistance  of  5 
ohms  and  L  =  i  henry.  The  impedance  at  60  ~  will  then  be 


2  +  (6.28  x  60  x  i)a  =  ^25  +  376.8'  =377.  8  ohms,  sub- 
stantially  the  same  as   the  inductance  alone,   and    under  an 


138  ELECTRIC   TRANSMISSION   OF  POWER. 

impressed  E.  M.  F.  of  1,000  volts  the  resulting  current  would 


be  2. 65  amperes.    But  tan  q>  = 


_  377-S  _ 


75.56.   Hence  <p  =89°  15' 


and  cos  cp  =  .013.  Therefore  while  the  apparent  energy  is 
2.65  x  1,000  =  2,650  watts  the  real  energy  is  only  2,650  x  .013 
watts  =  34+:  really  the  loss  due  to  heating  the  conductor. 
This  is  of  course  a  very  exaggerated  case,  as  it  takes  no 
account  of  the  energy  that  would  be  required  to  reverse  the 
magnetization  in  whatever  iron  core  the  apparatus  might  have. 
It  does,  however,  show  very  clearly  that  the  current  flowing 
depends  practically  on  the  inductance  and  very  little  on  the 
resistance,  and  that  the  angle  of  lag  is  so  great  that  the  dis- 


1 j 


FIG.  56. 

crepancy  between  apparent  and  real  energy  may  also  be  very 
great.  In  practice  cos  cp  may  fall  as  low  as  o.  i  on  single 
pieces  of  apparatus  and  ranges  up  under  varying  conditions 
of  load  to  .95  or  more. 

These  practical  considerations  naturally  raise  a  question  as 
to  the  effect  of  impedances  in  parallel.  The  joint  impedance 
of  two  impedances  in  series  must  first  be  discussed. 

The  resistance  of  two  resistances  in  parallel  is  of  course 
familiar.  If  R  =  2  ohms  and  R1  =  4  ohms,  then  their  joint 
resistance  is  the  reciprocal  of  the  sum  of  their  reciprocals, 
thus, 

(R  +  R*}  =     ,    l     ,   —  i  rt  ohms. 


We  have  seen,  however,  that  impedances  cannot  be  added 
in  the  ordinary  manner.  If  we  take  two  impedances  made  up 
respectively  of  ft  =  4,  /  =  3,  and  R'  =  6,  /'  =  3,  we  must  pro- 
ceed as  in  Fig.  56.  The  first  impedance  is  5,  the  second  6.70. 


SOME  PROPERTIES  OF  ALTERNATING   CIRCUITS.      139 

The  true  impedance  of  the  two  in  series  is  given  by  the  dotted 
lines  and  is  n.66,  not  11.70.  That  is,  the  impedances  must  be 
added  geometrically,  since  unless  cp  —  <p ^  the  arithmetical  sum 
of  the  impedances  does  not  represent  the  facts  in  the  case. 
Similarly,  while  it  is  perfectly  true  that  the  joint  impedance 
of  two  impedances  in  parallel  is  equal  to  the  reciprocal  of  the 
sum  of  their  reciprocals,  the  summation  must  be  done  as  in 
Fig.  56  to  take  account  of  the  difference  of  phase  which  may 
exist  in  the  two  branches.  Taking  the  data  just  given,  the 
reciprocals  of  the  two  impedances  are .  20  and  .  149  respectively. 
Drawing  these  on  any  convenient  scale  as  in  Fig.  57,  preserv- 
ing between  them  the  angle  due  to  the  difference  of  phase  as 
given  by  q>  and  tpjt  we  find  the  geometrical  sum  of  the  recipro- 


cals to  be  .348,  of  which  the  reciprocal  is  2.87.  This  is  the  joint 
impedance  of  the  two  which  we  have  thus  geometrically  added. 

This  same  process  can  be  extended  to  any  number  of  impe- 
dances in  parallel.  It  is  important  to  note  that  since  the  cur- 
rents in  such  cases  are  generally  not  in  phase  with  each  other, 
it  usually  happens  that  the  sum  of  the  currents  in  the  branches 
differs  from  the  current  in  the  main  circuit,  as  they  are 
ordinarily  measured.  At  any  particular  instant,  however,  there 
is  strict  equality  between  the  whole  current  and  its  parts,  the 
inequality  appearing  when  the  complete  cycles  with  their 
phase  differences  are  considered.  It  is  in  fact  a  prominent 
characteristic  of  alternating  circuits  that  both  currents  and 
voltages  are  liable  to  violate  all  the  traditions  of  continuous- 
current  practice  in  a  way  apparently  very  erratic.  Particularly 
is  this  the  case  when  there  is  capacity  in  the  circuit,  a  condi- 
tion which  we  will  now  investigate. 

By  a  circuit  having  capacity  we  mean  one  so  constituted  that 
E.  M.  F.  applied  to  it  stores  up  energy  in  the  form  of  electro- 
static stress,  which  starts  this  energy  back  in  the  form  of 
current  when  the  constraining  E.  M.  F.  is  removed. 

Such  a  condition  exists  whenever  two  conductors  are 
separated  by  an  insulating  medium,  or  dielectric,  as  in 


HO  ELECTRIC    TRANSMISSION  OF  POWER. 

the  ordinary  condenser  of  Fig.  58.  Here  A  and  B  are  two- 
metal  plates  separated  by  a  layer,  C,  of  some  insulating- 
material.  If  now  these  plates  are  connected  to  the  terminals. 
of  a  dynamo  they  become  electrostatically  charged.  The 
electrostatic  stress  tends  to  draw  the  plates  together  and  in 
addition  sets  up  intense  strains  in  the  dielectric  C,  rendering 
potential  thereby  a  certain  amount  of  energy  which  flows  into 
the  apparatus  in  the  form  of  electric  current.  This  energy  is 
returned  as  current  if  the  original  electromotive  stress  is. 
removed  and  A  and  B  are  connected  together.  The  medium 
behaves  just  as  if  it  were  a  strained  spring,  and  when  it  returns 
its  energy  to  the  circuit  it  does  so  spring-fashion  with  rapid 


FIG.  58. 

oscillations,  dying  out  the  more  slowly  the  less  resistance  they 
encounter. 

The  capacity  of  such  a  condenser  is  the  quantity  of  energy 
which  it  can  store  up  as  electrostatic  strains  in  C.  It  is. 
proportional  to  the  area  of  the  plates,  to  the  E.  M.  F.  produc- 
ing the  strains,  and  to  the  " dielectric  constant"  of  C,  that  is, 
the  coefficient  which  for  that  particular  substance  measures  its. 
power  to  take  up  electrostatic  strains.  Oddly  enough  the 
capacity  decreases  as  C  grows  thicker,  indicating  that  the 
intensity  of  the  strain  is  the  thing  which  counts  rather  than 
the  volume  of  dielectric.  Without  knowing  the  exact  charac- 
ter of  electrostatic  strain  it  is  difficult  to  get  a  clear  mechanical 
idea  of  the  state  of  things  which  causes  the  energy  stored  to 
increase  as  the  thickness  of  Cdiminishes.  A  similar  condition, 
however,  holds  for  a  wire  held  tightly  at  one  end  and  twisted 
at  the  other;  the  shorter  the  wire  the  more  energy  stored  for 
a  given  angle  of  twist. 

As  in  the  case  of  inductance,  for  practical  purposes  the  unit 
of  capacity  is  taken  in  terms  of  unit  pressure,  /.  <?.,  one  volt. 
Unit  capacity  then,  in  terms  of  energy,  is  the  capacity  of  con- 


SOME   PROPERTIES   OF  ALTERNATING   CIRCUITS.       14* 

denser  in  which  one  watt-second  can  be  stored  under  an  elec- 
tromotive stress  of  one  volt.  This  capacity  is  one  farad,  and 
as  it  is  many  thousand  times  larger  than  anything  found  in 
practice  joll  QUO  °f  ^  (tne  microfarad}  is  more  often  used. 

When  a  condenser  is  used  with  an  alternating  current  the 
rate  at  which  energy  is  stored  and  delivered  evidently  increases 
with  the  frequency,  or  what  is  the  same  thing,  for  a  given 
alternating  E.  M.  F.  the  greater  the  frequency  the  greater  the 
current  received  and  delivered  by  the  condenser. 

Numerically  the  current  in  a  condenser  of  capacity  k 
farads,  supplied  by  an  E.  M.  F.  of  <?  volts  at  n  cycles  per 
second  is 

C  =  2  7t  n  e  k  , 

which  is  simply  the  current  due  to  e  volts  and  k  farads 
multiplied  by  the  frequency  expressed  in  angular  measure. 
Thus  if  we  have  a  2  microfarad  condenser  fed  by  an  alternat- 
ing E.  M.  F.  of  2,000  volts  and  130  cycles  per  second  the 
current  flowing  is 

2,000  x  6.28  X  130  X  2 

C=-  -  =  3.26  amperes. 

1,000,000 

In  such  an  alternating  circuit  then  there  will  be  a  substantial 
current  flowing  in  spite  of  the  fact  that  there  is  a  break  in  the 
conductor  at  the  condenser.  In  short  the  circuit  acts  as  if  it 


had  a  resistance  of  -L—^  =  613  ohms,  which  is  the  impedance  of 

the  circuit.     More  exactly  the  impedance  is  -  ~.     It  should 

2  n  nk 

be  noted  here  that  some  writers  refer  to  this  fundamental 
condenser  function  (2  n  n)  k  as  capacity-speed.  Capacity-im- 
pedance really  is  a  negative  reactance,  often  termed  condensa?ice. 
To  see  the  relation  which  this  capacity-impedance  bears  to 
other  impedances  in  the  circuit  it  is  necessary  to  look  into  the 
properties  of  the  E.  M.  F.  of  the  condenser.  As  energy  is 
stored  in  the  condenser  the  opposing  stresses  in  it  increase 
until  the  applied  E.  M.  F.  can  no  longer  force  current  into  it 
and  the  condenser  is  fully  charged.  At  the  moment,  then. 
when  current  ceases  to  flow,  the  E.  M.  F.  of  the  condenser 
tending  to  discharge  it  is  at  a  maximum.  Hence  since  the 


142  ELECTRIC   TRANSMISSION  OF  POWER. 

one  has  a  maximum  as  the  other  is  zero,  the  E.  M.  F.  of 
the  condenser  and  the  charging  current  are  90°  apart  in 
phase. 

But  the  inductive  E.  M.  F.  is  also  90°  from  the  current,  and 
as  we  have  seen,  lagging.  It  has  its  maximum  when  the 
current  is  varying  most  rapidly,  and  when  the  strength  of  cur- 
rent in  a  given  direction  is  increasing,  the  inductive  E.  M.  F. 
in  the  same  direction  is  diminishing,  as  shown  in  Fig.  53.  As 
regards  capacity,  however,  the  moment  of  maximum  condenser 
E.  M.  F.  in  a  given  direction  is  that  at  which  the  current 
thereby  becomes  zero,  so  that  as  the  current  changes  sign  it 
has  behind  it  the  thrust  of  the  full  E.  M.  F.  of  the  discharging 
condenser,  while  at  the  same  moment  as  we  have  just  seen  the 
opposing  inductive  E.  M.  F.  is  at  its  maximum.  Hence  the 
E.  M.  F.  of  the  condenser  has  a  maximum  in  one  direction 
when  the  inductive  E.  M.  F.  has  its  maximum  in  the  other 
direction.  The  two  are  thus  180°  apart  in  phase,  and  each 
being  90°  from  the  current,  the  condenser  E.  M.  F.  must  be 
regarded  as  90°  ahead  of  the  current,  just  as  the  inductive 
E.  M.  F.  is  90°  behind  it. 

The  condition  of  affairs  is  shown  in  Fig.  59.  Here  a  a  is 
the  line  of  zero  current  and  E.  M.  F.  All  quantities  above 
this  line  may  be  regarded  as  -\- ,  and  all  below  it  as  —  ;  b  is  a  -f- 
wave  of  current  to  which  appertains  c  c  the  curve  of  inductive 
E.  M.  F.  lagging  90°  behind  the  current,  and  d  d  the  con- 
denser E.  M.  F.,  leading  the  current  90°. 

It  is  evident  that  these  two  E.  M.  Fs.  always  are  opposing 
each  other — when  one  is  retarding  the  current  the  other  is 
accelerating  it  and  vice  versa. 

The  condenser  E.  M.  F.  has  no  effect  on  the  total  energy 
of  the  circuit  for  the  same  reason  that  held  good  in  respect 
to  Fig.  53;  it  is  obviously  akin  to  a  spring,  alternately  receiv- 
ing and  giving  up  energy,  but  absorbing  next  to  none. 

Capacity  may  be  considered  as  negative  inductance  in  many 
of  its  properties.  If,  as  in  Fig.  59,  it  is  in  amount  exactly 
equivalent  to  the  inductance,  the  total  effect  on  the  circuit  is 
as  if  neither  capacity  nor  inductance  were  in  the  circuit.  In 
such  case  it  is  as  if  C  B,  Fig.  51,  should  be  reduced  to  zero. 
The  impressed  E.  M.  F.  then  becomes  equal  to  the  effective 
E.  M.  F.,  the  angle  of  lag  vanishes  and  the  circuit  behaves  as 


SOME  PROPERTIES  OF  ALTERNATING  CIRCUITS.      143 

if  it  contained  resistance  only.  If  the  condenser  E.  M.  F.  is 
not  quite  large  enough  to  annul  the  inductance  it  simply 
reduces  it. 

Fig.  60  illustrates  the  effect  of  varying  amounts  of  capac- 
ity.    In  the  main  triangle  A  B  C,  the  sides  have  the   same 

b 


FIG.  59. 

signification  as  in  Fig.  51.  Since  the  capacity  E.  M.  F.  is 
1 80°,  from,  i.  e.,  directly  opposite  to,  the  inductive  E.  M.  F.,  the 
effect  of  adding  the  capacity  E.  M.  F.  C  D,  is  to  reduce  the 
effective  inductance  to  B  D  and  give  as  an  impressed  E.  M.  F. 
A  D  and  an  angle  of  lag  cp^  Now  increasing  CD  to  equal  C  £, 
the  inductance  is  annulled,  (p  becomes  zero,  and  the  impressed 


FIG.  60. 

and  effective  E.  M.  Fs.  are  the  same.  Then  increase  C  D  still 
further  so  that  it  becomes  C  E.  Now  the  inductance  C  B 
not  only  is  neutralized  but  is  replaced  by  a  negative  inductance 
•B  E.  The  angle  of  lag  now  becomes  an  angle  of  lead,  <p^  the 
necessary  impressed  E.  M.  F.  rises  to  A  E,  and  the  circuit 
behaves  as  regards  the  relations  between  current,  E.  M.  F.,  and 
energy,  just  as  it  did  when  affected  by  inductance.  There  is 
the  same  discrepancy  between  real  and  apparent  energy,  the 


144 


ELECTRIC   TRANSMISSION  OF  POWER. 


same  necessity  for  more  current  to  represent  the  same  energy. 
But  adding  inductance  now  decreases  the  angle  of  lead.  From 
a  practical  standpoint  capacity  by  itself  is  objectionable,  but 
capacity  in  a  line  containing  inductance  is  sometimes  a  very 
material  advantage. 

The  nature  and  reality  of  this  curious  phenomenon  of 
"leading"  current  in  an  alternating  circuit  may  be  appre- 
ciated by  an  examination  of  Fig.  61.  This  shows  the  actual 
curves  of  current  and  E.  M.  F.  taken  from  a  dynamo  working 
on  a  condenser  in  parallel  with  inductance.  The  maximum  of 
the  current  wave  is  very  obviously  in  advance  of  the  maximum 
of  the  E.  M.  F.  wave,  though  by  a  rather  small  amount 


90 

FIG.  61. 


180 


(actually  about  6°).  The  capacity  in  this  case  was  between 
2  and  3  microfarads. 

Treating  capacity  as  a  negative  inductance  enables  us  to 
compute  its  effects  quite  easily.  We  have  already  seen  how 
to  reckon  the  impedance  of  a  condenser;  using  the  word 
impedance  here  in  its  proper  sense  of  apparent  resistance  by 
whatever  caused.  This  quantity  we  can  add  geometrically  to 
the  ohmic  resistance  of  a  circuit  and  obtain  the  net  impedance 
just  as  in  Fig.  54.  We  must  bear  in  mind,  however,  that  the 
capacity  E.  M.  F.  is  180°  from  the  inductance  E.  M.  F., 
though  each  is  at  right  angles  to  the  effective  E.  M.  F.  which 
is  concerned  with  the  ohmic  resistance. 

Instead     then    of     computing     the    total     impedance     as 


V  ^3  H-  /2,  it  becomes 


v~ 


\2    7T  n  I/I 


the  second  term  under 


the  radical  being  the  square  of  the  apparent  resistance  due  to 
the  capacity,  just  as  72  expressed  the  square  of  the  apparent 
resistance  due  to  inductance. 


SOME  PROPERTIES  OF  ALTERNATING   CIRCUITS.      145 

Suppose  for  example  we  have  a  resistance  of  100  ohms  in 
series  with  a  condenser  of  4  microfarads  capacity.  The 
impressed  E.  M.  F.  is  2,000  volts  at  130  cycles  per  second. 


What  is  the  total  impedance,  the  resulting  current,  and  the 
angle  cp,  in  this  case  an  angle  of  lead?     Here 


6.28  X  13°  X  4 
,.000,000         = 


Laying  off  the  resistance  A  B  in  Fig.  62  as  in  Fig.  54  and 


drawing 


to  the  same  scale  at  right  angles  (downward  to 


2  n  nk 

emphasize  its  opposition  to  the  inductance  of  Fig.  54),  we 
have  for  the  length  of  the  diagonal  A  C,  which  represents  the 
total  impedance,  ^loo3  -f  3o62  =  322  ohms.  The  current 
flowing  is  then,  6.21  amperes.  The  angle  (p  is  determined  as 

before  by  tan  cp  =  «= —  =  3.06,  whence  <p  =  72°,  cos  cp  =  .309, 
100 

so  that  we  are  dealing  with  a  "power  factor"  like  that  pro- 
duced by  a  heavy  inductance,  although  the  current  leads  the 
E.  M.  F.  instead  of  lagging  behind  it.  If  we  consider  an 


146  ELECTRIC   TRANSMISSION  OF  POWER. 

inductance  in  series  with  this  circuit,  we  should  have  to  reckon 
it  upward  in  Fig.  62,  thereby  subtracting  it  from  the  former 
length  B  C. 

Suppose  for  example  for  the  given  inductance  L  =  .3  henry. 
Then  7=2  7t  n  L  =  245.  If  in  Fig.  62  we  draw  245  on 
the  scale  already  taken,  upward  from  £7,  we  shall  reach  the 
point  D.  B  D  therefore  is  61,  and  A  Z>,  the  resulting  imped- 
ance, is  y'loo2  -f-  6ia  =  117  ohms.  The  new  current  is  there- 
fore —  -  =  17.09  and  as  tan  (pl  =  .61,  q>  t  =  31°. 5,  being  still 

an  angle  of  lead. 

It  is  easy  to  see  that  for  a  certain  value  of  /,  the  capacity 
effect  and  inductance  effect  would  exactly  balance  each  other. 

This  value  is  obviously  2  7t  n  L  =  T ,  since  then  in  Fig.   62, 

2  nnk 

BC — C  D  —  Q,  and  the  impedance  and  resistance  are  the 
same  thing,  while  cp  becomes  zero. 

In  actual  circuits  the  capacity  is  seldom  in  series  with 
the  inductance.  It  is  usually  made  up  of  the  aggregated 
capacity  of  the  line  wires  with  air  as  the  dielectric,  the  capac- 
ity of  any  underground  cables  that  may  be  in  circuit,  and 
finally  the  capacity  of  the  apparatus,  transformers,  motors,  and 
the  like,  that  may  be  in  circuit.  Generally  the  major  part 
of  the  total  inductance  is  in  the  apparatus  rather  than  the  line, 
and  hence  in  parallel  with  the  capacity.  In  many  cases  nearly 
all  the  inductance  and  capacity  is  due  to  the  apparatus,  and 
the  two  may  be  .regarded  as  in  parallel  substantially  at  the 
ends  of  the  line.  The  inductance  of  generators  and  trans- 
formers may  amount  to  several  henrys,  while  their  capacity  is 
by  no  means  small,  though  very  variable,  like  the  inductance. 
For  example,  the  capacity  of  a  large  high-voltage  generator  or 
transformer  may  often  amount  to  several  tenths  of  a  micro- 
farad. Armored  or  sheathed  cable  has  a  capacity  of  from  a 
quarter  to  a  half  microfarad  per  mile.  Altogether  one  may 
expect  to  find  a  capacity  of  several  microfarads  not  infrequently 
and  considerable  fractions  of  a  microfarad  very  often. 

Suppose  now  we  have  in  parallel  a  capacity  A,  Fig.  63,  of  2 
microfarads,  and  an  inductance  of  .5  henry,  the  resistance  con- 
nected with  each  being  insignificant.  Assuming  as  before  2,000 
volts  and  130  cycles,  what  is  the  total  impedance  of  the  com- 


SOME   PROPERTIES   OF  ALTERNATING  CIRCUITS.      147 

binatiori,  and  the  resulting  current?  We  have  already  seen 
how  impedances  in  parallel  are  to  be  treated.  In  the  case  in 

hand  the  impedance  of  A  is  ^ — : =  613  ohms, 

6.28  X  130  X  .000,002 

and  that  of  B  is  6.28  x  130  x  -5  =  4°8  ohms.  Now  remem- 
bering that  in  adding  impedances  their  geometrical  sum  is  to 
be  taken  and  that  joint  impedance  is  the  reciprocal  of  the 
geometrical  sum  of  the  reciprocals  of  its  components,  we  can 
proceed  as  follows:  The  reciprocal  of  613  is  .00163.  This  we 
will  lay  off  to  any  convenient  scale  just  as  in  Fig.  57.  As  it  is 
capacity-impedance  we  will  dr?»w  it  downward  for  the  sake  of 
uniformity,  making  A  B,  Fig.  64.  Now  take  the  inductance. 


Fis.  63. 

The  reciprocal  of  408  is  .00245.  As  the  inductance  and 
capacity  E.  M.  Fs.  are  here  as  before  at  an  angle  of  180°,  we 
must  draw  this  upward  from  B,  giving  us  the  distance  B  C. 
The  geometrical  sum  is  then  A  C  =  .00082,  of  which  the  recip- 
rocal gives  the  resultant  impedance  as  1,219  ohms.  Hence 

the  net  current  in  the  line   under  2,000  volts  is  — =  1.6 

1,219 

ampere.  But  under  the  same  pressure  the  current  in  A  would 
obviously  be  3.26  amperes  and  that  in  the  inductance  B  would 
be  4.90  amperes.  We  have  then  the  curious  phenomenon  of 
a  total  current  in  the  line  smaller  than  that  through  either  of 
the  two  impedances  in  circuit.  It  is  as  if  A  and  B  formed  a 
local  circuit  by  themselves  in  which  the  condenser  A  served 
as  a  species  of  generator.  It  is  quite  evident  that  the  total 
energy  of  the  system,  however,  is  that  due  to  the  current  in 
the  line,  so  that  the  phases  in  A  and  B  are  greatly  displaced. 
If  the  resistances  in  the  circuit  were  quite  negligible  the  net 
current  in  the  line  would  be  indefinitely  small  when  A  =  B, 

that  is,  when  Z  =  -.  Of  course,  however,  the  true  impedances 
of  both  A  and  ,#are  modified  by  the  resistances,  however  small, 


148 


ELECTRIC   TRANSMISSION  OF  POWER. 


so  that  in  Fig.  64  the  impedances  will  always  be  at  a  small  angle 
with  the  E.  M.  Fs.  instead  of  being  coincident.  Hence  the  net 
current  can  never  become  zero,  though  when  the  impedances 
of  A  and  B  are  large  compared  with  the  resistances,  the  line 

current  will  be  very  small  when  L  —  -. 

k 

This  case  is  in  sharp  contrast  to  that  in  which  condenser 
.and  inductance  are  in  series  with  each  other.     For  then  the 

line  current  is  increased  as  L  approaches  —  instead  of  becom- 

K 

ing  smaller  relatively  to  the  branch  currents,  although  in  each 
case  the  same  relation  between  capacity  and  inductance  gives 


B 
FIG.  64. 

the  maximum  "  power  factor  "  on  the  circuit,  since  whatever 
the  current,  under  this  condition  it  depends  most  nearly  en  the 
resistance  alone.  When  the  resistance  is  quite  perceptible  in 
comparison  with  the  impedances  of  A  and  B,  we  should  form  a 
resultant  impedance  with  each,  and  then  combine  the  two 
somewhat  as  in  Fig.  56. 

If  then  we  have  an  inductive  load  of  any  kind  in  circuit,  a 
condenser  in  parallel  therewith  will  reduce  the  current  on  the 
line  and  thereby  increase  the  "  power  factor"  of  the  system. 
It  does  this,  too,  without  any  material  loss  of  energy  and  with- 
out necessarily  increasing  the  amount  of  current  flowing  through 
the  inductance  under  a  given  E.  M.  F.  on  the  line.  Were 
condenser  and  inductance  in  series  the  power  factor  could 
likewise  be  improved  up  to  a  certain  point,  but  trouble  would 
be  encountered  in  that  the  condenser  would  necessarily  have 


SOME  PROPERTIES  OF  ALTERNATING   CIRCUITS.      *49 

to  be  large  enough  to  let  pass  enough  current  to  supply  the 
energy  required  in  the  inductance  at  full  load. 

In  all  practical  cases  the  relations  between  resistance, 
capacity  and  inductance  which  have  just  been  set  forth,  are 
somewhat  modified  by  the  existence  of  losses  of  energy  in  the 
circuit  quite  apart  from  these  due  merely  to  overcoming  of 
resistance.  Energy  is  required  to  reverse  the  magnetization 
of  the  iron  cores  of  inductance  coils,  and  to  reverse  the  electric 
strains  in  the  dielectric  of  condensers.  It  therefore  happens 
that  with  a  condenser  in  circuit  the  condenser  current  is  not 


Fia  65. 


exactly  90°  ahead  of  its  impressed  E.  M.  F.,  as  shown  in  Fig. 
59>  but  a  trifle  less,  so  that  the  current  has  a  small  component 
in  phase  with  its  E.  M.  F.,  thus  supplying  the  energy  in 
question.  The  deviation  from  90°  is  generally  but  a  small 
fraction  of  a  degree.  The  same  sort  of  thing  happens  when 
an  inductance  having  an  iron  core  is  in  circuit.  However 
small  the  resistance,  the  lag  still  misses  90°  by  enough  to  take 
account  of  the  energy  required  for  magnetic  losses.  The 
variation  from  90°  in  this  case  may  amount  to  30°  or  more. 
Hence  the  failure  to  take  account  of  these  energy  losses  in 
the  example  given  on  page  137. 

It  therefore  comes  to  pass  that  in  practically  figuring  the  re- 
lations between  resistance,  capacity  and  inductance,  one  appar- 
ently deals  with  oblique-angled  triangles.  The  result  is  that 
in  adding  inductance  and  capacity  effects  one  seems  not  to  get 
so  simple  results  as  in  Fig.  64,  but  something  more  like  Fig. 
65.  Here  it  is  clear  that  no  combination  of  capacity  and  induct- 
ance can  leave  the  circuit  free  from  everything  except  resist- 
ance, for  both  the  inductance  and  the  capacity  demand  energy 


150  ELECTRIC    TRANSMISSION   OF  POWER. 

in  the  circuit  beyond  that  expended  in  the  resistance.  Evi- 
dently, however,  cp  may  be  reduced  to  zero  if  the  relation 
between  capacity  and  inductance  is  just  right.  Thus  while 
the  lag  may  be  reduced  to  zero,  no  combination  can  dodge 
the  energy  losses. 

Closely  connected  with  this  subject  is  the  matter  of  res- 
onance, which  will  be  taken  up  in  connection  with  the  discus- 
sion of  the  line.  Briefly  the  phenomenon  is  this:  We  have 
seen  that  the  E.  M.  F.  of  a  condenser  is  a  maximum  when  the 
current  is  zero,  so  that  as  the  current  changes  sign  the  thrust 
of  the  condenser  E.  M.  F.  is  behind  it.  Now  if  the  condenser 
E.  M.  F.  synchronizes  with  this  current,  the  impressed  E.  M.  F. 
is  added  to  it,  imposes  an  added  stress  on  the  condenser  dur- 
ing the  next  alternation,  catches  therefrom  an  additional  kick 
as  it  passes  through  zero  again,  and  so  on.  Thus  the  net 
effective  E.  M.  F.  is  raised  by  the  action  of  the  condenser 
and  would  increase  enormously  but  for  its  being  frittered 
away  in  overcoming  resistance  and  supplying  such  energy 
losses  as  we  have  just  been  considering.  By  avoiding  these 
losses  as  far  as  possible  one  can  actually  raise  the  voltage  on 
an  alternating  circuit  to  twenty-five  or  thirty  times  its  nominal 
amount  by  employing  a  condenser  of  the  proper  capacity. 
Even  when  the  impressed  E.  M.  F.  and  the  current  are  not 
quite  in  phase,  one  has  always  a  component  of  the  condenser 
E.  M.  F.  tending  to  act  in  a  similar  manner.  Whether  it 
actually  produces  a  sensible  rise  of  voltage  depends  on  its  rela- 
tions to  the  frequency  and  resistance  with  which  it  has  to  deal. 
In  fact  it  is  the  addition  of  this  same  condenser  E.  M.  F.  to 
the  circuit  that  enables  one  to  neutralize  inductive  E.  M.  F. 
Whether  or  not  the  neutralization  of  inductance  by  capacity 
produces  a  real  resonant  rise  of  voltage  depends  on  the  fre- 
quency and  whether  the  energy  losses  are  small  or  large.  If 
they  are  small  enough  to  let  the  sum  of  the  impressed  and  the 
condenser  E.  M.  F.  accumulate  during  several  alternations- 
there  will  be  a  noticeable  increase  of  voltage,  otherwise  not. 

The  dynamics  of  resonance  may  perhaps  be  best  understood 
by  a  very  pretty  mechanical  analogue  due  to  Dr.  Pupin.  The 
apparatus  on  which  it  is  based  is  shown  in  Fig.  66.  It  is  a 
torsional  pendulum  composed  of  a  heavy  bar  A  suspended  by 
a  stiff  elastic  wire  B,  from  a  light  circular  bearing  plate  C 


SOME   PROPERTIES  OF  ALTERNATING  CIRCUITS.      151 

This  plate  rests  in  a  recess  a,  with  a  frictional  resistance 
which  can  be  regulated  by  the  screw  shown  in  the  cut.  Such 
an  apparatus  acts  much  like  an  electric  circuit,  having  induc- 
tance, capacity  and  ohmic  resistance.  The  moment  of  inertia 
of  the  bar  A  corresponds  to  self-induction,  the  elasticity  of 
B  to  condenser  capacity  as  we  have  just  noted  in  connection 
with  Fig.  58,  and  the  friction  of  C  to  the  resistance.  More- 
over, if  7  is  the  moment  of  inertia  of  the  bar  A,  and  B  the 
reciprocal  of  the  elastic  capacity  of  the  wire,  then  within  cer- 
tain values  of  the  frictional  resistance  the  oscillation  period  of 
the  pendulum  thus  formed  is,  in  seconds, 

This  corresponds  most  beautifully  to  the  time  constant  of  an 
electric  circuit,  which  is,  if  the  energy  losses  are  within  cer- 
tain limits, 

27T 


1000 


wherein  L  is  in   henrys,  C  the  capacity  in  microfarads,  and 
the  denominator  comes  from  the  units  being  thus  chosen. 


FIG.  66. 

Now  if  this  pendulum  be  given  a  twist  it  will  oscillate  at 
constant  frequency  until  the  friction  gradually  brings  it  to 
rest  with  oscillations  of  steadily  decreasing  amplitude.  If, 
however,  at  the  end  of  each  complete  swing  it  should  receive  a 
slight  push,  its  oscillations  would  continue  and  would  increase 
in  amplitude  up  to  a  limit  set  by  the  frictional  resistance. 


152  ELECTRIC    TRANSMISSION  OF  POWER. 

The  condition  for  such  permanent  increase  of  amplitude  is 
that  fat  frequency  of  the  pushes  must  coincide  with  the  period 
of  the  pendulum.  In  the  electrical  case,  resonance  thus 
occurs  when  the  frequency  and  the  time  constant  of  the  cir- 
cuit are  equal.  Further,  maintaining  our  auxiliary  pushes  at 
their  original  frequency,  suppose  /  to  be  decreased  by  taking 
weight  off  A  progressively.  As  the  time  constant  of  the 
pendulum  thus  diminished  a  point  would  be  found,  and  that 
very  soon,  at  which  resonance  would  cease,  and  the  same 
result  would  follow  increase  of  /,  so  that  when  the  circuit 
begins  to  get  out  of  tune  the  resonance  soon  becomes  rather 
trivial.  If,  however,  the  pushes  were  supplemented  by  others 
°f  3>  5»  7>  etc-»  times  the  frequency,  corresponding  to  the 
harmonics  found  in  an  ordinary  alternating  circuit,  new  points 
of  resonance  would  appear  when  the  period  of  A  assumed 
corresponding  values. 

As  to  the  magnitude  of  the  resonant  effect,  in  the  torsional 
pendulum  case  the  amplitude  evidently  increases  with  the 
strength  of  the  pushes,  their  absolute  frequency,  which  meas- 
ures the  energy  supplied,  and  the  moment  of  inertia  of  A, 
which  stores  this  energy.  It  decreases  in  virtue  of  the  fric- 
tional  resistance.  Corresponding  reasoning  holds  in  the  elec- 
trical case,  and  to  a  first  approximation  the  E.  M.  F.  in  a 
resonant  circuit  is 

17'          H    L    f? 

^ 


in  which  E  is  the  impressed  E.  M.  F.  concerned,  L  the 
inductance  in  henrys,  R  the  resistance  in  ohms,  and  n  the 
frequency.  In  case  of  resonance  with  harmonics,  n  and  E 
refer  to  the  frequency  and  magnitude  of  the  harmonic  impli- 
cated, and  E  becomes  a  resonant  component  of  the  E.  M.  F. 
wave.  This  subject  will  be  discussed  more  at  length  in 
Chap.  XII. 

We  have  now  looked  into  the  most  important  characteristics 
of  alternating  currents  —  those  concerned  with  the  phenomena 
of  inductance  and  capacity. 

It  remains  to  note  very  briefly  some  other  physical  peculiar- 
ities that  are  of  practical  importance. 

The  most  important  single  property  of  alternating  current 


TRANSMISSION  BY  ALTERNATING   CURRENTS.        153 

is  the  ease  with  which  it  can  be  changed  inductively  from  one 
voltage  to  another.  If  a  circuit  carrying  such  a  current  is  put 
in  inductive  relation  with  another  circuit  as  in  Fig.  4,  Chap.  1, 
the  electromagnetic  stresses  set  up  by  the  first  circuit  can  be 
utilized  to  produce  alternating  current  of  any  desired  voltage 
in  the  second  circuit.  The  details  of  the  operation  will  be 
taken  up  later;  suffice  it  to  say  here  that  it  is  essentially  the 
transformation  of  the  electromagnetic  energy  due  to  one 
circuit  into  electrical  energy  in  another  circuit. 

Alternating  currents  can  be  regulated  in  amount  by  putting 
inductance  in  the  circuit  without  losing  more  than  a  very 
trifling  amount  of  energy.  This  very  property,  however,  is 
troublesome  when  an  alternating  current  is  used  for  magnetiz- 
ing purposes.  It  is  very  difficult  to  get  a  large  current  to  flow 
around  a  magnet  core  because  of  the  high  inductance,  and  even 
then  the  magnetic  and  other  losses  in  the  core  are  serious  unless 
great  care  is  taken.  These  difficulties  have  stood  in  the  way 
of  getting  a  good  alternating  motor  until  within  the  past  few 
years,  and  even  now  such  motors  have  to  be  designed  and  con- 
structed with  the  greatest  care  to  avoid  trouble  from  induct- 
ance and  iron  losses.  For  some  classes  of  work,  such  as  teleg- 
raphy and  electrolytic  operations,  the  alternating  current  is 
ill  suited  save  under  special  conditions  and  with  special  appar- 
atus. For  the  general  purposes  of  electrical  power  transmis- 
sion it  is  singularly  well  fitted,  from  the  great  ease  with  which 
transformations  of  voltage  can  be  made,  certain  very  valuable 
properties  of  the  modern  alternating  motor,  and  the  great 
simplicity  and  efficiency  with  which  regulation  can  be  effected. 
In  addition  there  is  some  reason  to  believe  that  with  equally 
good  regulation  of  the  voltage,  incandescent  lamps  are  subject 
to  slightly  less  deterioration  on  an  alternating  circuit  than 
when  worked  with  continuous  current.  This  difference  be- 
comes very  conspicuous  in  the  case  of  the  recently  introduced 
Nernst  lamp. 


CHAPTER  V. 

POWER   TRANSMISSION    BY    ALTERNATING    CURRENTS. 

BROADLY  considered,  we  may  say  that  all  systems  of  trans- 
mitting power  by  alternating  currents  are  closely  akin  in 
principles  and  characteristics.  The  growth  of  the  art,  how- 
ever, has  proceeded  along  several  lines,  and  certain  conven- 
tional distinctions  have  come  to  be  observed  in  considering 
the  methods  employed  for  rendering  the  alternating  current 
applicable  to  the  working  conditions  of  power  transmission. 

Alternating  systems  are  usually  classified  as  either  mono- 
phase or  polyphase.  By  the  former  term  is  generally  under- 
stood a  system  generating,  transmitting  and  utilizing  a  simple 
alternating  current  such  as  shown  in  diagram  in  Fig.  44.  By 
the  latter  is  meant  a  system  generating,  transmitting  and 
utilizing  two  or  more  such  currents  differing  in  phase  and 
combined  in  various  ways.  As  regards  the  systems,  this  dis- 
tinction is  sufficiently  sharp,  but  as  regards  individual  parts  of 
such  systems  the  line  of  demarcation  is  sometimes  hazy,  since 
a  monophase  current  may  be  the  source  of  derived  polyphase 
currents,  and  on  the  other  hand  polyphase  currents  may  be  so 
combined  as  to  give  a  monophase  resultant.  Mixed  systems 
involving  unsymmetrical  phase  relations  may  properly  be 
called  heterophase. 

As  regards  apparatus,  any  device  that  performs  all  its  func- 
tions in  a  normal  manner  when  deriving  all  its  energy  from  a 
simple  alternating  current  should  be  classified  as  monophase. 
If  its  functions  require  the  co-operation  of  energy  received 
from  two  or  more  alternating  currents  differing  in  phase, 
the  apparatus  is  essentially  polyphase. 

For  certain  purposes  the  one  system  is  best  adapted,  for 
certain  other  purposes  the  other  is  most  advantageous,  but 
the  underlying  principles  are  the  same,  and  the  apparatus  has 
much  the  same  general  properties. 

The  material  of  alternating  transmission  work  may  be  classi- 
fied as  follows,  the  transmission  line  itself  being  reserved  for 

154 


TRANSMISSION  BY  ALTERNATING   CURRENTS.        155 

discussion    in  another  chapter  in  connection  with  other  line 
work: 

I,   Generators.  III.   Synchronous  Motors. 

II.  Transformers.  IV.   Induction  Motors. 

After  a  tolerably  careful  examination  of  the  practical  prop- 
erties of  this  apparatus  in  its  various  forms,  we  shall  be  able 
to  appreciate  its  application  to  the  electrical  transmission  of 
power  under  various  circumstances.  Subsidiary  apparatus  of 
all  kinds  will  be  referred  to  in  its  proper  place,  and  the  divers 
systems  that  have  been  exploited  can  best  be  considered  after 
we  have,  in  a  cold-blooded  sort  of  way,  looked  into  the  char- 
acteristics of  their  component  parts. 

Alternating  power  transmission  is  now  going  through  the 
stage  of  development  that  is  inseparable  from  the  rise  of  a 
•comparatively  new  art — the  planting  time  of  "systems,"  if 
one  may  be  allowed  the  simile.  A  little  later  we  shall  be  able 
better  to  judge  the  quality  of  the  crop.  It  is  sufficiently 
-certain  already  that  the  same  sort  of  plant  will  not  do  equally 
well  under  all  circumstances. 

The  principles  of  the  alternating  current  dynamo  have 
already  been  explained,  but  the  constructional  features  of  such 
.machines  are  sufficiently  distinct  from  those  of  continuous 
current  dynamos  to  warrant  examination  in  considerable 
detail. 

The  modifications  peculiar  to  alternators  are  in  general  due 
to  two  causes;  first,  the  general  use  of  a  fairly  high  frequency, 
and,  second,  the  necessities  of  rather  high  voltage. 

We  have  already  seen  that,  while  an  ordinary  continuous 
current  dynamo  fitted  with  collecting  rings  will  give  alternat- 
ing current,  the  frequency  is  rather  low.  To  secure  a  higher 
frequency  it  becomes  necessary  to  increase  the  number  of  poles, 
the  speed,  or  both.  Increasing  the  number  of  the  poles  is 
the  usual  method  employed,  since  continuous  current  dynamos 
are  generally  for  the  sake  of  keeping  up  the  output  operated  at 
speeds  as  high  as  the  conditions  of  economical  use  render 
desirable.  So  we  usually  find  that  for  equal  outputs  alternators 
have  many  more  poles.  The  general  relation  between  poxes, 
speed  and  frequency  is, 


'56 


ELECTRIC    TRANSMISSION   OF  POWER. 


where  p  is.  the  number  of  poles,  N  the  revolutions  per  min- 
ute, and  n  the  complete  cycles  per  second. 

For  example,  belt-driven  continuous  current  dynamos  of  100 
to  500  kilowatts  usually  run  at  speeds  from  600  down  to  300, 
and  have  four  or  six  poles,  thus  giving  15  to  20  cycles  per  sec- 
ond, while  modern  alternators  of  similar  size  and  speed  have 
from  12  to  24  poles,  thus  adapting  them  for  a  frequency  of 
30-^  to  6o~.  Machines  for  the  older  frequencies  of  i2o~  to 
i4o~  were  usually  even  more  liberally  provided  with  poles 
unless  driven  at  speeds  considerably  above  those  mentioned. 
The  general  appearance  and  design  of  a  typical  modern  alter- 
nator is  shown  in  outline  in  Fig.  67.  This  is  a  I5O-KW  gener- 


FIG.  67. 

ator  running  at  600  revolutions  per  minute,  and  shows  admir- 
ably the  general  characteristics  of  rather  numerous  poles,  low 
base,  and  massive  bearings  that  nowadays  belong  in  common 
to  machines  by  nearly  all  makers.  Such  alternators  usually 
have  very  powerful  field  magnets,  and  the  projecting  pole- 
pieces  are  usually  built  up  of  iron  plates  like  the  armature,  for 
the  same  purpose  of  preventing  eddy  currents  in  the  iron. 
The  ring  of  field  magnets  is  split  on  the  level  of  the  centre  of 


TRANSMISSION  BY  ALTERNATING  CURRENTS.       157 

the  shaft,  for  convenience  in  removing  the  armature.  The 
weight  of  belt  driven  generators  of  the  output  named  is  usu- 
ally in  the  neighborhood  of  seven  tons. 

This  same  general  type  is  adhered  to  whatever  the  nature 
or  voltage  of  the  armature  winding,  save  in  the  case  of  special 
machines. 

The  winding  of  a  modern  alternator  is  nearly  always  widely  dis- 
tinct from  continuous-current  windings.  In  alternators  the  volt- 
age is  generally  from  1000  volts  up,  seldom  below  500  volts,  and 
to  obtain  this  the  windings  corresponding  to  the  numerous  poles 
are  almost  universally  connected  in  series  instead  of  in  parallel. 

This  necessitates  specially  connecting  the  armature  coils 
in  a  very  characteristic  way.  For  when  a  given  armature 


FIG.  68. 

coil  is  approaching  one  of  the  north  poles  of  the  field  magnet 
and  is  generating  current  in  a  given  direction,  the  next  arma- 
ture coil  is  necessarily  approaching  the  neighboring  south  pole, 
and  if  wound  in  the  same  direction  as  the  first  coil  would 
generate  a  current  flowing  in  the  opposite  direction.  Hence 
if  all  the  armatures  coils  are  to  be  in  series,  they  must  be 
wound  alternately  in  opposite  directions,  as  shown  in  Fig.  68. 
This  arrangement  throws  in  series  the  E.  M.  Fs.  generated 
by  all  the  armature  coils.  Sometimes  for  convenience  the 
halves  of  the  armature  are  connected  in  parallel,  thus  giving 


IS8  ELECTRIC   TRANSMISSION  OF  POWER. 

half  the  voltage  and  twice  the  current  by  a  simple  change  in 
connections.  Fig.  69  shows  in  diagram  such  a  winding  for  a 
i6-pole  field,  and  its  relation  to  the  collecting  rings.  Note 
that  each  half  of  the  winding  preserves  the  characteristics 
shown  in  Fig.  68. 

In  practical  machines  as  built  to-day,  the  armature  coils  are 
nearly  always  bedded  in  slots  in  the  armature  core.  The 
early  American  machines  were  generally  built  with  smooth 
armature  cores,  and  upon  these  flat  coils  were  laid  and  held  in 
place  by  an  elaborate  system  of  binding  wires.  This  construc- 
tion has  been  virtually  abandoned  by  all  the  principal  manu- 


FIG.  69. 

facturers  in  favor  of  the  so-called  "  iron-clad  "  armature,  which 
has  the  double  advantage  of  great  mechanical  solidity  and  of 
permitting  the  armature  coils  to  be  wound  in  forms  thoroughly 
insulated,  and  then  dropped  into  place  in  their  slots  and 
firmly  wedged  in  position.  The  winding  is  therefore  very 
little  liable  to  damage  and  easily  replaced  if  necessary.  .  . 

The  slotted  armature  cores  are  variously  arranged  in  dif- 
ferent machines,  but  always  with  the  same  object  in  view. 

Fig.  70  shows  one  widely  used  arrangement  of  slots.  Here 
the  coils  are  wound  in  forms  and  thoroughly  insulated.  They 
are  then  pushed  into  place  in  the  previously  insulated  slot, 
each  coil  enclosing  a  single  armature  tooth.  When  firmly  in 
place  the  insulating  material  is  put  into  position  above  them 
and  a  hard  wood  wedge  is  driven  into  the  dove-tailed  upper 
portion  of  the  slot,  holding  the  coils  and  their  surrounding 


TRANSMISSION  BY  ALTERNATING  CURRENTS.       159 

insulation  permanently  in  place.  The  coils  here  shown  con- 
sist of  only  four  turns  of  heavy  wire.  Often  there  are  many 
more  turns  per  coil,  and  sometimes  the  round  wire  is  replaced 
by  rectangular  bars.  For  use  with  raising  transformers  each 
coil  sometimes  consists  of  a  single  turn  of  bar  copper,  but 
whatever  the  nature  of  the  coil  the  slots  are  arranged  much 
as  shown  below. 

Another  excellent   form  of  slotted  armature    is  shown   in 
Fig.     71.       The  coils    are,    as   in    the  case    just    mentioned, 


FIG.  70. 

wound  in  forms  and  solidly  insulated.  They  are  then  sprung 
over  the  armature  teeth  into  place  and  tightly  wedged.  The 
slots  are  carefully  insulated  also,  and  by  the  time  the  winding 
is  completely  assembled  it  is  so  thoroughly  insulated  that 
repairs  are  few  and  far  between.  The  special  peculiarity  of 
this  form  of  core  is  that  the  outer  corners  of  the  teeth  are  cut 
away,  so  that  the  coils  come  more  gradually  into  the  field  of 


FIG.  71. 


the  pole-pieces  than  if  the  edges  were  sharp.  The  object  of 
this  device  is  to  obtain  a  curve  of  E.  M.  F.  more  nearly 
according  with  the  sine  wave  form,  and  experience  shows  that 
the  plan  works  successfully.  Without  such  precautions  the 
E.  M.  F.  curve  is  very  likely  to  be  quite  irregular,  and  even 
with  them  it  is  generally  none  too  smooth.  The  pole-pieces 
of  alternators  are  very  often  rounded  off  or  chamfered  away 
for  the  same  purpose. 

Nearly  all  modern  alternating  windings  are  like  those  just 


160  ELECTRIC    TRANSMISSION  OF  POWER. 

indicated,  of  the  drum  type.  The  Gramme  winding  is  seldom 
or  never  employed,  as  it  is  hard  to  wind  and  repair  and  has, 
for  alternators,  no  compensating  advantages.  Nor  has  the 
flat  coil  winding  without  iron  core  found  a  permanent  place  in 
American  practice,  although  it  is  widely  used  abroad. 

The  flat  coil  is  well  exemplified  in  the  form  introduced  by 
Ferranti,  of  which  the  armature  is  shown  in  Fig.  72.  The  thin 
coils  revolve  in  the  space  between  two  opposite  crowns  of  field 
magnets.  The  winding  is  connected  as  in  Fig.  69,  which  not 
only  enables  the  same  armature  to  be  reconnected  for  two 
voltages,  but  if  extraordinary  voltages  are  to  be  attempted, 


FIG.  72. 

keeps  armature  coils  of  widely  differing  potential  far  apart  in  the 
structure.  The  Ferranti  type  of  armature,  being  without  iron 
core,  has  a  very  low  inductance,  which  property  is  often  highly 
desirable.  There  is,  however,  considerable  likelihood  of  eddy 
currents  in  the  armature  conductors  unless  they  are  individu- 
ally very  thin,  and  for  this  and  obvious  mechanical  reasons 
American  designers  have  adhered  to  the  iron-clad  armature,. 


TRANSMISSION  BY  ALTERNATING   CURRENTS.        161 

which  is  admirable  mechanically  and  magnetically,  and  have 
taken  other  means  to  get  out  of  the  difficulty  of  its  high 
inductance. 

As  in  other  dynamos,  the  theoretical  E.  M.  F.  generated 
by  an  alternator  depends  on  the  strength  of  the  magnetic 
field,  the  number  of  armature  conductors  under  induction,  and 
the  speed  at  which  they  are  driven  through  the  field.  As  an 
alternator  receives  load  the  E.  M.  F.  at  its  terminals  is  reduced 
by  three  several  causes. 

First,  there  is  a  loss  of  voltage  due  to  energy  lost  in  the 
armature  conductors.  This  depends  simply  on  the  current 
and  resistance  and  is  numerically  equal  to  CR. 

Second,  there  is  self-induction  in  the  armature  windings, 
which,  as  we  have  already  seen,  involves  an  inductive 
E.  M.  F.,  lagging  90°  behind  the  impressed  E.  M.  F.  The 
effect  of  this  is  to  partly  neutralize  the  impressed  E.  M.  F., 


FIG.  73. 

as  in  all  cases  of  inductance.  The  amount  of  this  disturb- 
ance depends  on  the  frequency,and  the  magnetic  relation  of 
the  armature  coils  to  each  other  and  to  the  field  magnets. 
This  relation  of  course  varies  according  to  the  relative  posi- 
tion of  the  armature  teeth  which  carry  the  coils.  In  Fig.  73, 
purposely  shown  with  somewhat  exaggerated  teeth,  the  arma- 
ture is  in  the  position  of  minimum  inductance,  for  the  mag- 
netic field  set  up  by  the  armature  coils  is  not  here  much 
strengthened  by  the  presence  of  the  pole-pieces.  If,  however, 
the  armature  were  shifted  forward  or  backward  so  that  each 
tooth  would  be  just  opposite  a  pole-piece,  the  field  from  the 
armature  coils  would  traverse  an  almost  complete  loop  of  iron 


162 


ELECTRIC   TRANSMISSION  OF  POWER. 


and  the  inductance  of  the  armature  would  be  a  maximum.  In 
this  position  the  armature  teeth  might  be  almost  as  good 
magnet  poles  as  the  field  poles  themselves;  at  all  events,  con- 
secutive teeth  would  be  united  by  an  almost  continuous  iron 
core,  and  the  armature  inductance  would  be  very  high.  ,  '. 

One  of  the  best  ways  of  reducing  this  inductance  and  its 
train  of  troubles  is  to  make  the  magnetization  due  to  the  field 
magnets  as  strong  as  is  practicable.  This  not  only  utilizes  the 
iron  of  the  field  magnets  and  armature  to  the  best  advantage, 
but,  so  to  speak,  pre-empts  its  power  of  receiving  magnetiza- 
tion so  that  the  current  about  the  armature  teeth  finds  a  poor 
field  for  its  inductive  operations.  In  addition,  this  strengthen- 
ing of  the  field  enables  the  required  E.  M.  F.  to  be  obtained 


FIG.  74. 

with  fewer  turns  per  tooth.  This  of  itself  is  a  great  advan- 
tage, since  increasing  the  number  of  turns  in  an  iron-cored 
coil  runs  up  the  inductance  with  appalling  rapidity.  A  glance 
at  Fig.  74  will  show  the  reason  why.  Suppose  we  have  a 
looped  iron  core  wound  with  four  turns  of  wire,  a,  £,  <r,  d. 
If  we  pass  a  certain  alternating  current  around  two  turns, 
a  and  £,  we  shall  have  a  certain  inductance  due  to  the  reaction 
of  the  change  in  magnetism  on  these  two  coils.  Now  pass 
the  same  current  around  all  four  coils.  The  magnetization 
will  be  approximately  doubled  and  the  number  of  turns  on 
which  it  acts  will  also  be  doubled.  That  is,  each  coil  is  acted 
upon  by  double  the  force  and  there  are  twice  as  many  total 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       163 

coils.  Hence  the  total  inductance  will  be  about  four  times  as 
great  as  at  first,  and  in  general  it  will  increase  with  the  square 
of  the  number  of  turns.  If,  however,  as  just  suggested,  the 
core  is  nearly  saturated  already,  adding  the  two  extra  turns, 
f  and  d,  will  not  anywhere  nearly  double  the  magnetization, 
since  iron  already  magnetized  responds  less  and  less  to 
additional  magnetizing  force  as  this  force  increases. 

Hence,  as  we  shall  see  later,  diminishing  the  number  of 
armature  turns  that  can  act  conjointly  in  producing  effective 
magnetization  lowers  the  inductance  very  rapidly. 

The  third  disturbing  cause  which  tends  to  reduce  the 
effective  E.  M.  F.  of  an  alternator  is  the  reaction  of  the 
armature  current,  through  the  resulting  magnetization,  on 
the  field  magnets.  We  have  already  seen  that  when  a  closed 
coil  is  driven  into  and  out  of  a  magnetic  field  the  induced 
current  is  always  in  such  direction  as  to  cause  work  to  be 
done  in  driving  the  coil.  But,  since  the  current  due  to  enter- 
ing the  field  is  equal  and  opposite  to  that  produced  in  leaving 
the  field,  the  total  magnetizations  due  to  these  currents  are 
equal  and  opposite,  and  if  one  opposes  the  field  due  to  a  pole- 
piece  the  other  will  in  an  equal  degree  strengthen  that  field. 
Hence,  provided  these  two  actions  are  applied  alike;  /'.  <?., 
are  symmetrical  with  respect  to  the  field,  the  total  effect  of 
armature  current  will  be  neither  to  weaken  nor  strengthen 
the  field. 

In  practice  the  effect  of  the  armature  reaction  is  two-fold. 
If  the  current  be  nearly  in  phase  with  the  E.  M.  F.  the  main 
result  of  the  magnetic  field  set  up  by  the  armature  is  to 
distort  that  due  to  the  field  without  greatly  weakening  it  as 
a  whole.  The  result  of  this  distortion  is  that  the  E.  M.  F. 
does  not  increase  and  decrease  steadily  following  a  sine  wave, 
but  becomes  irregular.  The  working  E.  M.  F. ,  as  measured 
on  a  voltmeter,  changes  but  a  trifle,  but  the  maximum. 
E.  M.  F.  becomes  subject  to  great  variations.  Fig.  75  shows 
in  a  very  striking  manner  the  result  of  field  distortion  from 
a  purely  non-inductive  load.  Here  a  is  the  E.  M.  F.  curve 
on  open  circuit  and  b  is  the  curve  as  modified  by  the  armature 
reaction  at  nearly  full  load.  The  arrow  shows  the  direction 
of  rotation  of  the  armature.  In  this  case  the  maximum 
voltage  was  increased  about  30  per  cent.,  while  the  measured 


164 


ELECTRIC    TRANSMISSION  OF  POWER. 


voltage  was  nearly  constant.  Bearing  In  mind  that  the 
E.  M.  F.  at  any  moment  is  due  to  the  rate  of  change  of  the 
magnetic  induction  through  the  armature,  and  not  to  the 
absolute  amount  of  that  induction,  it  is  tolerably  obvious  that 
the  effect  of  field  distortion  due  to  armature  reaction  may 
vary  widely  according  to  the  shape  and  position  of  both 
the  pole-pieces  and  the  armature  teeth.  It  may  increase  the 
maximum  voltage  as  above,  or  decrease  it  fully  as  much,  but 
if  it  is  of  any  considerable  magnitude  it  always  deforms 
the  E.  M.  F.  wave  very  materially. 

If,  however,  through  armature  inductance  or  inductive  load 
the  current  lags  behind  the  E.M.F.,  we  have  a  very  different 
state  of  affairs.  The  current  reaches  its  maximum  after  the 


500 


400 


300 


200 


100 


FIG.  75. 

armature  coil  has  passed  beyond  the  position  of  maximum 
E.  M.  F.,  and  the  net  magnetization  produced  by  it  chokes 
back  the  field,  at  the  same  time  greatly  distorting  it. 

If  the  only  effect  of  armature  reaction  and  inductance  were 
to  cause  a  loss  of  voltage  there  would  be  little  cause  for  alarm. 
But  as  shown  in  Fig.  75,  the  E.  M.  F.  wave-shape  often  un- 
dergoes profound  changes,  which  may  greatly  increase  the 
chance  for  serious  resonance.  As  already  noted,  alternating 
generators,  monophase  and  polyphase  alike,  give  in  practice  an 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       165 

E.  M.  F.  wave  which  is  not  sinusoidal,  but  contains  the  odd 
harmonics  of  the  fundamental  frequency.  These  are  a  neces- 
sary result  of  the  variations  in  magnetic  reluctance  and  arma- 
ture reactance  when  the  armature  is  in  various  angular 
positions,  as  well  as  of  subsidiary  reactions  in  transformers 
and  other  apparatus.  The  harmonics  of  even  order  do  not 
appear,  since,  unless  a  machine  is  deliberately  made  unsym- 
metrical,  all  the  variations  in  E.  M.  F.  are  complete  within 
each  half  period,  the  second  half  of  the  cycle  merely  showing 
a  reversal  of  sign.  Hence,  only  those  harmonics  appear  which 
pass  through  one  or  more  complete  cycles  in  a  half  period  of 
the  fundamental,  /.  £.,  by  construction  all  the  harmonics  are 
of  odd  order.  These  harmonics  have  a  very  real  existence, 


and  can  readily  be  identified  by  testing  electrically  for  reso- 
nance, or  even  by  hunting  for  them  with  a  telephone  in  some 
cases.  By  taking  the  wave  form  of  the  machine  by  the  con- 
tact method  or  photographically,  the  nature  and  magnitude  of 
the  harmonics  are  at  once  made  evident. 

Fig.  76  shows  the  wave  form  of  a  machine  that  was  carefully 
studied  by  Steinmetz.  It  is  from  a  three-phase  generator  hav- 
ing but  one  armature  tooth  per  phase  per  pole,  and  giving  150 
K\V  at  LOOO  volts  and  6o~.  Curve  A  is  the  E.  M.  F.  wave  of 


i66 


ELECTRIC    TRANSMISSION  OF  POWER. 


one  coil  to  the  common  connection,  at  no  load,  B  is  the  wave 
as  calculated  from  a  summation  of  the  harmonics  up  to  the 
fifteenth,  and  C  shows  the  residual  traces  of  still  higher  har- 
monics. To  reduce  the  vertical  scale  to  primary  volts, 
multiply  by  10.  Analysis  of  this  wave  showed  that  it  cor- 
responded approximately  to  the  following  equation: 

Sin  a  —  .12  sin  (30  —  2.3)  —  .23  sin  (50  —  1.5) 
-j-  .134  sin  (70  —  6.2). 

In  other  words  the  third  harmonic  has  about  12  per  cent.,  the 
fifth  about  23  per  cent.,  and  the  seventh  about  13  percent,  of 
the  amplitude  of  the  fundamental. 

At  full  load  the  shape  of  this  wave  is  changed  in  a  most  sin- 
gular manner.  The  armature  reaction  shifts  the  magnitudes 
and  positions  of  the  variations  in  the  magnetic  field  and  of  the 
harmonics  due  to  them.  Fig.  77  shows  the  wave  form  from 


£_ 

\ 

120 

\ 

1 

\ 

100 

so 
I  *° 

70 

«: 

j 

/ 

^ 

^. 

^~ 

\ 

/ 

\ 

, 

\ 

/ 

\ 

/ 

\ 

/ 

/ 

\ 

•:SO 

SO 

10 
0 

—10 

-wv 

1 

/ 

\ 

/ 

\ 

/ 

\ 

/ 

s~ 

•»-^ 

^ 

N 

>^ 

jT 

/ 

/o 

10 

1 

JO 

40 

50 

0 

70 

80 

'JO 

1 

K) 

10 

120 

130 

1 

0 

150 

100 

17 

/ 

FIG.  77. 

this  machine  under  load.  The  central  depression  of  Fig.  76 
is  replaced  by  a  slight  hollow  between  a  high  peak  and  a  shoul- 
der, and  the  wave  is  conspicuously  unsymmetrical,  as  might 
readily  be  predicted  from  the  general  effect  of  the  armature 
reaction.  The  approximate  equation  to  the  wave  of  Fig.  77  is 

Sin  a  —  .  176  sin  (3^  -)-  1 1.7)  —  .085  sin  (50  —  33'8) 
-|-  .01  sin  (70  -j-  26.6). 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       167 

The  effect  of  the  armature  reaction  due  to  load  has  been 
greatly  to  strengthen  the  third  harmonic,  greatly  to  weaken 
the  fifth,  and  nearly  to  suppress  the  seventh. 

Obviously  changes  of  this  sort  may  have  a  very  great  effect 
in  the  matter  of  resonance.  Suppose,  for  example,  that  the 
conditions  on  the  line  at  light  load  were  such  as  to  give 
marked  resonance  with  the  seventh  harmonic  of  the  frequency. 
Now,  under  all  ordinary  working  conditions  this  harmonic 
would  be  practically  absent;  but  if  a  large  part  of  the  load  were 
thrown  off,  resonance  would  suddenly  appear,  and  with  the 
lessened  armature  reaction  the  general  voltage  would  rise 
sharply,  so  that  serious  results  might  follow.  In  case  of  a  high 
voltage  generator,  say  for  10,000  volts,  having  the  curves  just 
given,  at  load  the  seventh  harmonic  would  only  have  an  ampli- 
tude of  about  100  volts,  while  this  amplitude  would  suddenly 
rise  to  1340  volts,  increased  perhaps  four  or  five  times  by 
resonance,  when  the  load  was  thrown  off.  Under  other  condi- 
tions throwing  on  load  might  produce  an  equally  unpleasant 
effect. 

Change  in  wave  form  at  varying  load  is  for  the  most  part 
chargeable  to  armature  reaction,  like  the  accompanying  change 
of  voltage,  so  that  for  stability  of  performance  under  changes 
of  load  a  generator  must  have  low  armature  reaction  and 
small  armature  inductance.  These,  however,  do  not  neces- 
sarily imply  a  sinusoidal  wave  form,  which  may  be  given  at 
light  load  by  a  machine  of  fairly  high  reaction  and  inductance. 
But  if  a  generator  is  to  give  a  wave  form  closely  sinusoidal  at 
all  loads,  it  must  have  these  constants  small,  particularly  if  it 
is  expected  to  perform  well  under  inductive  load,  which  pro- 
duces the  severest  distortional  effects  upon  the  magnetic  field. 

The  magnetizing  and  demagnetizing  effects  of  the  arma- 
ture current  in  case  of  inductive  lag  no  longer  can  balance 
each  other,  for  they  are  unsymmetrical  with  respect  to  the 
poles.  If  the  angle  of  lag  is  large  the  result  will  be  a 
very  serious  weakening  of  the  field,  and  a  correspondingly 
large  drop  in  the  effective  voltage.  For  example,  a  certain 
alternator  of  120  KW  output  has  40  turns  of  wire  per  arma- 
ture tooth,  carrying  a  normal  full  load  current  of  60 
amperes.  There  is  thus  a  possible  demagnetizing  force  of 
2400  ampere-turns  at  full  load.  The  ampere  turns  per  pole- 


i68 


ELECTRIC    TRANSMISSION  OF  POWER. 


piece  in  the  same  machine  are  3,600,  so  that  if  the  current 
should  lag  enough  to  give  the  armature  reaction  full  play,  as 
might  happen  from  excessive  armature  inductance  alone,  the 
total  net  magnetizing  force  would  be  reduced  to  a  third  of 
its  normal  amount  and  the  resulting  voltage  to  a  half  or  less. 
It  is  in  fact  common  enough  to  find  alternators  that  require 
from  50  to  100  per  cent,  increase  in  the  exciting  ampere- 
turns  to  hold  them  at  normal  voltage  under  a  full  load  current 
lagging  even  15°  or  20°. 

Between    inductance   and    armature    reaction   the   effective 
E.  M.  F.  of  alternators  generally  falls  off  rapidly  under  load, 


130 


120 


110 


100 


90 


10 


30 


50 


60 


AMPERES 

FIG.  78. 


unless  special  care  be  taken  with  the  design.  The  loss  from 
ohmic  resistance  is  usually  trivial  compared  with  those  just 
named.  It  is  in  fact  perfectly  practicable  to  build  an  alternator 
with  inductance  and  armature  reaction  so  exaggerated,  that  a 
very  slight  increase  in  current  will  cut  down  the  voltage  so 
rapidly  as  to  keep  the  current  virtually  constant.  This  plan 
was  successfully  carried  out  in  the  remarkable  Stanley  alter- 
nating arc  machine  of  a  few  years  ago. 

In  this  case  the  current  varied  only  about  10  per  cent,  while 
the  voltage  varied  between  a  few  volts  and  over  2,000.  An 
automatic  short-circuiting  switch  was  provided  to  avert  dan- 
gerous rise  of  voltage  in  case  of  an  accidental  open  circuit. 

In  so-called  constant  potential  alternators,  as  usually  built, 
the  inherent  regulation  is  by  no  means  good.  Fig.  78  gives 
an  excellent  idea  of  the  performance  of  some  of  the  earlier 


TRANSMISSION  BY  ALTERNATING   CURRENTS.        169 

machines  in  this  respect,  and  it  is  about  what  one  would  find 
in  many  alternators  now  in  service,  except  for  their  compound 
winding. 

It  has  often  been  held  that  high  inductance  and  large  arma- 
ture reaction  are  desirable  in  alternators  in  order  to  prevent 
burn-outs  in  case  of  accidental  short  circuits.  While  it  is  per- 
fectly true  that  sufficiently  crude  armature  design  does  produce 
this  effect,  by  limiting  the  possible  current,  it  is  equally  true 
that  a  machine  with  sufficient  inductance  and  reaction  to  serve 
as  a  practical  safeguard  will  regulate  so  atrociously  as  to  be 
under  most  circumstances  incapable  of  decent  commercial 
service  under  present  conditions.  When  it  was  sufficient  for 
an  alternator  to  give  current,  that  with  sufficient  hand  regula- 
tion could  supply  house  to  house  transformers  most  of  the 
time,  high  inductance  machines,  which  are  easy  and  cheap  to 
build,  answered  the  purpose. 

At  present,  when  the  importance  of  good  regulation  is  gener- 
ally understood,  and  most  large  alternating  plants  must  look 
forward  to  assuming  a  motor  load,  low  inductance  machines 
with  small  armature  reaction  are  essential  for  first  class  service. 
For  power  transmission  plants  with  heavy  mixed  loads  of 
lights  and  motors,  no  other  class  of  machine  should  be  toler- 
ated, or  can  be  used  without  incessant  annoyance. 

Most  even  of  the  older  alternators  are  compound-wound 
to  compensate  for  armature  effects,  and  are  thus  enabled  to 
work  successfully  up  to  outputs  at  which  the  voltage  begins  to 
fall  off  too  fast  to  be  thus  compensated.  So  long  as  the  com- 
pounding process  actually  gives  good  regulation,  it  is  useful 
and  enables  the  generators  to  be  worked  at  a  high  output.  As 
a  matter  of  fact  when  used  with  generators  of  the  older  type, 
even  compounding  left  much  to  be  desired.  As  alternating 
practice  has  gradually  improved,  compound-wound  alternators 
have  been  more  skillfully  designed,  and  recent  machines  give 
on  non-inductive  load  a  very  fair  approximation  to  constant 
potential.  Fig.  79  shows  the  E.  M.  F.  of  a  modern  over-com- 
pounded alternator  at  varying  load.  If,  however,  the  current 
has  even  a  moderate  lag  behind  the  E.  M.  F.,  owing  to  induc- 
tance in  the  machine  or  the  load,  the  machine  will  no  longer  give 
constant  potential,  and  the  voltage  may  fall  off  rapidly  as  the 
load  comes  on,  as  shown  in  the  cut.  The  reason  for  this  we  have 


170  ELECTRIC   TRANSMISSION   OF  POWER. 

already  found  in  the  extra  increase  of  field  excitation  necessary 
to  compensate  for  the  demagnetizing  effect  of  armature 
reaction.  Incidentally  if  the  current  commuted  to  supply  the 
series  field  lags  much,  the  process  of  commutation  cannot  go 
on  normally  without  adjusting  the  brushes  to  compensate  for 
the  lag. 

Therefore  for  inductive  load  the  compounding  has  to  be 
greatly  increased,  and  even  then  is  correct  only  for  a  particu- 
lar inductance. 

It  must  be  understood  that  alternators  are  compounded  on  the 
same  general  principles  as  continuous  current  machines,  except 


1200 


1100 


1000 


INDUCTjygfQ 


25  50  75  100 

KILOWATTS  OUTPUT 

FIG.  79- 


150 


that  instead  of  the  current  for  the  series  winding  being  derived 
from  the  general  commutator  of  the  dynamo,  it  is  generally 
obtained  from  a  simple  special  commutator.  A  shunt  around 
this  commutator  diverts  most  of  the  main  current,  while  a  por- 
tion is  rectified  and  passed  around  the  fields.  Fig.  80  shows  in 
diagram  a  common  compounding  arrangement.  The  two  col- 
lecting rings  A  and  B  with  the  commutator  Care  mounted  on 
the  armature  shaft.  Brushes  on  A  and  B  take  off  the  alternat- 
ing current.  One  of  these  rings,  A,  leads  directly  to  line.  The 
current  going  to  the  other  ring  is  divided,  part  passing  around 
C  through  the  resistance  box  D,  and  part  being  rectified  by 
the  commutator  for  use  in  the  series  field.  This  commutator 
has  as  many  segments  as  there  are  pairs  of  poles  in  the  field, 
the  alternate  sections  being  electrically  united. 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       171 

By  varying  the  resistance  Z>,  the  amount  of  current  diverted 
into  the  field  can  be  varied  and  the  compounding  may  thus  be 
arranged  to  keep  the  voltage  constant  at  the  terminals  or  at  any 
point  on  the  line.  A  similar  change  in  D  may  be  made  to 
adjust  the  compounding  for  inductive  load  of  any  given  power 
factor. 

For  non-inductive  loads,  or  for  inductive  loads   of  constant 


FIG.  80. 

power  factor,  this  compounding  gives  good  results,  but  for  a 
load  of  widely  varying  power  factor  it  is  simply  worthless 
unless  supplemented  by  hand  regulation. 

If  compounding  is  to  be  successfully  used  for  keeping  con- 
stant potential  on  a  circuit  of  lights  and  motors  subject  to 
considerable  variations  in  the  power  factor,  it  must  be  applied 
to  a  generator  of  very  low  inductance  and  armature  reaction. 
Otherwise  no  adjustment  of  the  compounding  for  any  particu- 


172 


ELECTRIC    TRANSMISSION  OF  POWER. 


lar  power  factor  will  give  approximately  constant  potential 
when  the  power  factor  varies. 

For  example  it  would  be  hopeless  to  attempt  to  compound 
in  the  ordinary  way  an  alternator  having  a  characteristic 
like  Fig.  78,  so  that  it  would  be  tolerable  on  a  commercial  cir- 
cuit of  lights  and  motors.  On  the  other  hand,  a  generator 
having  a  voltage  characteristic  like  Fig.  81  could  readily  be 
so  compounded.  Here  the  fall  in  voltage  at  constant  field 
excitation,  from  no  load  to  full  load  (non-inductive),  is 
about  3^  per  cent.  Under  inductive  load  this  fall  would 


2400 
02300 


2100 


2000 


10 


20 


30 


40         50          60 

OUTPUT  IN  K.W 

FIG.  81. 


TO 


80 


90        100- 


be  increased  considerably,  but  from  the  usual  ratio  of 
inductive  drop  to  armature  reaction  found  in  the  best 
modern  generators,  the  variation  for  the  power  factors  likely 
to  be  encountered  with  a  mixed  load  would  be  somewhat 
smaller  than  the  original  drop.  The  total  variation  from  no 
load  to  full  inductive  load  would  then  be  between  6  and  7  per 
cent.,  and  with  compounding  adroitly  adjusted  for  average  con- 
ditions the  greatest  variation  from  normal  voltage  could  easily 
be  brought  within  2  per  cent.  A  little  intelligent  hand  regu- 
lation at  certain  times  of  the  day  would  improve  even  this 
good  result. 

These  considerations  apply  to  polyphase  as  well  as  to  mono- 
phase generators.  The  advent  of  polyphase  work  has  done 
much  to  improve  all  alternators,  and  especially  with  respect  to 
regulation. 

The  generation  of  polyphase  alternating  currents  is  a  very 
simple  matter.  The  object  in  view  is  the  production  of  two  or 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       175 

more  similar  currents  differing  in  phase  by  some  convenient 
amount,  usually  60°  or  90°.  To  obtain  two  currents  90°  apart 
in  phase,  it  is  only  necessary  to  clamp  together  the  shafts  of 
two  common  alternators,  so  that,  for  a  construction  like  Fig. 
70,  the  slots  of  one  armature  would  be  opposite  the  teeth  of 


FIG.  82 


the  other  armature.  The  armatures  would  then  give  currents 
90°  apart  in  phase.  Such  combination  alternators  were  built  for 
the  Columbian  Exposition  by  the  Westinghouse  Company,  and 
were  used  for  the  principal  lighting  and  power  circuits.  These 
structures  are,  however,  expensive  for  the  output  obtained,  and 
the  two  windings  are  nearly  always  put  on  a  single  armature 
core,  and  spaced  as  just  described.  Fig.  82  shows  a  very  simple 


FIG.  83. 

winding  of  this  character.  There  are  four  times  as  many 
armature  slots  as  there  are  field  poles.  Each  coil  spans  two 
teeth.  The  coils  shown  by  solid  lines  form  one  phase  winding, 
the  dotted  coils  the  other  phase  winding.  Each  set  of  coils  is 
connected  as  an  ordinary  monophase  winding,  and  the  terminals 
are  brought  out  to  two  pairs  of  collecting  rings.  Such  a  wind- 
ing gives  two  simple  alternating  currents  related  in  phase  as 
shown  in  Fig.  83.  The  armature  core  is  very  fully  occupied 


174  ELECTRIC   TRANSMISSION   OF  POWER. 

by  the  two  windings,  rather  more  advantageously  than  it  could 
be  by  a  single  winding,  so  that  the  machine  gives  a  somewhat 
better  output  as  a  two-phaser  than  would  be  possible  with  a 
simple  alternator  of  the  same  dimensions.  And,  what  is  of 
more  importance,  the  regulation  of  the  machine  as  a  two-phaser 
is  much  better  than  it  would  be  as  asingle-phaser.  In  the  first 
place  the  armature  inductance  is  greatly  reduced  by  the  dis- 
tribution of  the  windings  and  the  reduction  of  the  ampere- 
turns  per  armature  tooth.  Second,  the  same  causes  act  to  cut 
down  the  armature  reaction  in  case  of  a  lagging  current. 
Anything  that  improves  the  intrinsic  regulation  also  means 
greater  output  for  unimproved  regulation.  Moreover,  the  in- 
creased number  of  armature  teeth  gives  a  more  uniform  reluc- 
tance than  in  the  case  of  fewer  teeth,  and  hence  tends  to  give 
a  better  approximation  to  a  sinusoidal  wave  form. 

So,  aside  from  the  value  of  polyphase  currents  for  motor  pur- 
poses, which  we  shall  presently  examine,  polyphase  winding  is 
valuable  on  its  own  account  as  increasing  output  and  improv- 
ing regulation.  In  fact  diphase  windings  were  devised  for  this 
purpose  before  their  importance  in  the  operation  of  motors 
became  generally  known. 

The  value  of  a  subdivided  winding  in  reducing  inductance 
and  armature  reaction  was  greatly  emphasized  by  the  intro- 
duction of  polyphase  generators,  and  it  was  a  short  step  from 
windings  like  Fig.  82,  having  one  coil  and  virtually  one  tooth 
per  phase  per  pole,  to  windings  in  which  each  phase  winding  is 
split  up  into  several  sets  of  coils  in  adjacent  slots,  thereby 
still  further  decreasing  the  effective  inductance  and  armature 
reaction.  Such  windings  may  be  called  polyodontal,  from  their 
several  teeth  per  phase  per  pole,  and  are  very  generally  used  in 
the  best  recent  machines.  A  fine  example  of  this  class  of 
winding  is  shown  in  Fig.  84.  This  is  a  quarter  section  of  the 
armature  of  one  of  the  5,000  HP  Niagara  generators,  showing  a 
portion  of  one  coil  belonging  to  a  single  phase.  The  full 
winding  is  composed  of  two  conductors  per  slot,  half  the  total 
slots,  in  alternate  groups,  belonging  to  each  phase. 

Such  complete,  subdivision  of  the  co.ils  results  in  low  induc- 
tance and  a  very  low  armature  reaction.  A  similar  winding  could 
be  used  for  a  monophase  generator,  and  will  have  to  be  em- 
ployed if  monophase  machines  come  to  be  used  extensively  for 


TRANSMISSION  BY  ALTERNATING   CURRENTS. 


75 


power  transmission  purposes.  The  form  of  armature  slot  used 
for  polyodontal  windings  is  shown  in  Fig.  85,  a  single  segment 
of  one  of  the  core  plates  of  the  armature  of  the  Niagara  two- 
phaser.  The  appearance  of  one  of  these  great  machines  com- 
plete is  admirably  shown  in  the  frontispiece,  showing  the 


FIG.  84. 

interior  of  the  Niagara  station.  The  field  magnets  are  re- 
volved instead  of  the  armature,  although  they  are  exterior  to 
it.  A  very  powerful  fly-wheel  effect  is  gained  by  this  arrange- 
ment, since  the  weight  of  the  revolving  structure,  turning  at 
250  r.  p.  m.,  is  about  75  tons,  half  of  this  being  in  the  field 


FIG.  85. 

itself.  This  is  about  12  feet  in  diameter,  a  single  forged  steel 
ring  with  twelve  massive  pole-pieces  secured  to  its  inner  face. 
The  normal  voltage  of  the  machine  is  about  2,250,  and  the 
frequency  is  25 ~.  The  stationary  armature  is  provided  with 
six  ample  ventilating  ducts,  through  which  air  is  forced  by 


176 


ELECTRIC   TRANSMISSION   OF  POWER. 


the  revolving  field.  Fig.  86  shows  a  vertical  section  of  the 
whole  apparatus  with  its  shaft  and  upper  bearings.  A  hun- 
dred and  forty  feet  below  the  generator  is  the  turbine  which  sup- 
ports by  hydraulic  pressure  the  weight  of  the  revolving  mass, 
save  a  ton  or  two  of  residual  weight,  which  may  be  either  posi- 
tive or  negative,  and  which  is  taken  care  of  by  a  thrust  bearing. 
The  full  load  of  this  generator  is  775  amperes  on  each  of  the 


FIG.  86. 

two  circuits,  and  at  this  load  the  commercial  efficiency  is  very 
nearly  97  per  cent. — the  highest  figure  yet  touched  by  any 
kind  of  generator.  The  exciting  current  for  the  fields  is 
derived  from  a  rotary  transformer,  and  is  led  into  the  revolving 
magnets  through  a  pair  of  collecting  rings  shown  in  Fig.  86  at 
the  extreme  top  of  the  shaft.  The  armature  current  is  of 
course  taken  from  stationary  binding  posts.  Altogether  the 
Niagara  machine  is  a  magnificent  specimen  of  polyphase 
construction. 


TRANSMISSION  BY  ALTERNATING   CURRENTS.        1 77 

When  three-phase  currents  instead  of  two-phase  are  to  be 
generated,  separate  armatures  are  out  of  the  question,  and  a 
winding  similar  to  that  of  Fig.  82  is  frequently  employed.  To 
obtain  the  three  currents,  however,  three  separate  windings  are 
employed,  arranged  as  in  Fig.  87.  The  coils  are  connected 


FIG.  87. 

so  that  a,  a,  a,  etc.,  form  one  phase  winding,  b,  b,  etc.,  a 
second,  and  c,  cy  etc.,  the  third.  The  close  similarity  of  this 
winding  to  the  two-phase  shown  in  Fig.  82  is  at  once  apparent. 
It  is  worth  noting  that  these  three  windings  are  spaced  60° 
•apart,  instead  of  90°,  as  in  a  winding  for  two  phases.  Naturally 


FIG.  8ya. 

therefore  the  currents  generated  would  be  different  in  phase 
by  only  60°,  giving  the  arrangement  of  currents  shown  in 
Fig.  873.  This  is  homologous  with  the  two-phase  current 
•system  of  Fig.  83. 

In    practice    it   is    necessary,    however,    to    have   the   sym- 


i78 


ELECTRIC    TRANSMISSION  OF  POWER. 


metrical  arrangement  of  phases  given  by  three  similar  cur- 
rents 120°  apart.  This  is  very  easily  obtained  in  the  external 
circuit  by  winding  one  set  of  the  armature  coils  in  a  direc- 
tion reversed  from  the-  other  two,  or  by  merely  reversing  the 
terminals  in  making  connections.  The  result  of  this  is  a  true 
three-phase  current,  such  as  is  shown  in  diagram  in  Fig.  88. 
It  has  now  the  curious  property  that  at  all  times  the  system  is 
simultaneously  carrying  currents  substantially  equal  in  both 
directions,  as  will  readily  appear  from  inspection  of  the  curves. 
With  such  a  current  it  is  usual  to  combine  the  circuits  cor- 


FIG.  88. 

responding  to  the  several  armature  windings.  Otherwise  we 
would  be  compelled  to  deal  with  circuits  of  six  wires,  and  the 
generator  would  have  six  collecting  rings. 

Moreover,  the  distribution  circuits  formed  by  combining 
the  circuits  as  just  indicated  have  the  advantage  of  economy 
in  copper,  as  we  shall  presently  see.  Hence,  the  three-phase 
system  has  become  the  mainstay  of  electrical  power  transmis- 
sion so  far  as  the  principal  circuit  is  concerned.  The  genera- 
tors may  be  two-phase  and  the  distributing  circuits  two-phase 
when  convenience  dictates,  but  the  main  line  is,  save  in  very 
rare  instances,  worked  three-phase.  The  change  from  two- 
phase  to  three-phase,  or  the  reverse,  is  accomplished  in  a  beau- 
tifully simple  and  efficient  manner,  to  be  described  later. 
Under  certain  circumstances  the  use  of  a  two-phase  generator 
has  at  least  the  theoretical  advantage  that  the  currents  in 
the  respective  armature  windings,  being  in  quadrature,  can 
have  little  or  no  mutual  reaction,  so  that  the  two  phases  are 
more  independent  than  the  three  phases  of  a  three-phaser. 


N  BY  ALTERNATING   CURRENTS.        i?9 

As  might  be  expected,  the  subdivision  of  windings  in  a  three- 
phase  armature  results  in  small  inductance  and  armature  reac- 
tion, smaller  in  fact  than  would  be  found  in  a  similar  two-phase 
winding.  Nevertheless  experience  shows  that  if  the  armature 
has  only  a  single  coil  per  phase  per  pole,  the  reaction  is  too 
great  for  first-class  regulation,  and  the  curve  of  E.  M.  F.  is 
rather  too  wide  a  departure  from  the  sine  wave.  It  is  quite 
usual,  therefore,  to  adopt  the  polyodontal  construction  with 
from  two  to  four  coils  per  phase  per  pole.  A  machine  carefully 


tic 

LSO 

120 

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10      80     30      *U      M     60      2Q      80      W    100    1] 

FIG.  89. 

JO    120    130    liO    ISO   J60    170  18 

designed  on  these  lines  can  be  made  to  give  excellent  regulation, 
with  voltage  not  varying  more  than  3  or  4  per  cent,  from  no 
load  to  full  non-inductive  load,  and  is  capable  of  giving  a  very 
close  approximation  to  a  true  sinusoidal  wave,  a  valuable 
characteristic  for  long  distance  transmission.  Fig.  89  shows 
the  wave  form  given  by  one  of  these  polyodontal  three-phasers. 
The  full  curve  shows  the  actual  E.  M.  F.,  the  dotted  line  the 
corresponding  sine  curve,  and  the  irregular  line  at  the  base  of 
the  figure  the  difference  between  the  two. 

There  are  several  methods  of  connecting  a  three-phase  wind- 


l8o  ELECTRIC   TRANSMISSION  OF  POWER. 

ing  to  its  external  circuit.  The  two  chiefly  used  are  generally 
know  as  the  "  star  "  and  "  mesh  "  connections.  In  the  former 
one  end  of  each  of  the  three  windings  is  brought  to  a  common 
junction,  and  the  three  remaining  ends  are  connected  to  three 
line  wires.  The  three  lines  then  serve  in  turn  as  outgoing  and 

A 


FIG.  90. 

return  circuits,  the  maximum  current  shifting  in  regular  rota- 
tion from  one  to  the  others  in  succession.  The  three  E.  M.  Fs. 
in  the  three  coils  differ  in  phase  by  120°,  owing  to  the  reversal 
of  which  we  have  spoken.  We  may  draw  the  star  connection 
diagrammatically  in  Fig.  90,  drawing  the  three  coils  a  b  c  120° 
apart  to  show  the  relation  of  the  E.  M.  Fs.  and  currents, 
although  they  lie  on  the  armature  as  shown  in  Fig.  87.  Three 


FIG.  91. 

of  the  terminals  meet  at  the  point  o,  the  others  are  connected 
respectively  to  the  lines  A,  B,  C.  As  the  three  windings  on  the 
armature  are  alike,  the  E.  M.  Fs.  generated  by  the  three  coils 
are  equal.  So  if  each  winding  #,  b,  c,  is  designed  for  1,000 
volts,  that  will  be  the  voltage  between  the  point  o  and  each  of 
the  three  lines  A,  B,  C.  Clearly,  however,  the  voltage  between 
any  two  of  these  lines,  as  A  and  B,  is  a  very  different  matter, 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       l8l 

since  it  results  from  the  addition  of  the  voltages  of  a  and  b,  which 
are,  however,  120°  apart  in  phase.  They  must  then  be  added 
geometrically.  Now  the  chord  of  120°  is  Vs  times  the  radius, 
so  that  the  geometrical  sum  of  the  voltages  a  and  &,  120°  apart, 
is  1.732  times  either  of  them.  The  voltage  then  between  A  and 
B  in  the  case  in  hand  is  1,732.  The  same  is  evidently  true  of 
the  other  pairs  of  lines  B,  C,  and  C,  A. 

The  other  ordinary  three-phase  connection  is  the  mesh,  in 
which  the  six  terminals  of  the  three  coils  are  united  two  and 
two,  and  the  lines  are  connected  to  the  three  points  of  junc- 
tion. This  arrangement  is  shown  diagrammatically  in  Fig.  91. 
Here  each  coil  must  generate  the  full  E.  M.  F.  between  any 
two  of  the  lines,  but  the  current  in  any  line,  as  B,  is  made  up  of 
the  geometrical  sum  of  the  currents  in  a  and  b,  differing  in 
phase,  just  as  the  E.  M.  F.  between  lines  in  Fig.  90  was  made 
up  of  the  sum  of  two  E.  M.  Fs.  The  current  in  B  being  then 
so  constituted,  is  <\/3  times  the  current  in  a  or  b,  and  so  on  for 
the  other  lines.  In  the  mesh  connection  we  deal  with  result- 
ant currents  just  as  in  the  star  we  find  resultant  E.  M.  Fs. 

An  armature  designed  for  a  given  working  voltage,  measured 
in  the  ordinary  way  between  lines,  would,  if  planned  for  star 
connection,  have  fewer  turns  of  larger  wire  than  if  intended  for 
mesh  connection.  This  is  sometimes  convenient,  and  is  useful 
in  keeping  the  voltage  between  coils  low.  The  mesh  connec- 
tion on  the  other  hand  has  more  turns  of  smaller  wire,  as  the 
current  is  diminished  while  the  E.  M.  F.  in  each  coil  is  the  full 
E.  M.  F.  between  lines.  This  property  is  useful  under  certain 
conditions,  as  it  makes  the  E.  M.  F.  between  any  two  lines  some- 
what less  dependent  on  the  actions  going  on  in  the  other  pairs 
of  lines.  The  same  windings  can  of  course  be  connected  either 
star  or  mesh,  according  to  the  dictates  of  convenience.  Both 
these  combination  circuits  have  in  common  one  immensely 
valuable  property.  They  require  for  the  transmission  of  a  given 
amount  of  energy  at  a  given  percentage  of  loss,  only  75  per  cent, 
of  the  weight  of  copper  required  for  the  same  transmission  at 
the  same  working  voltage,  by  continuous  current  or  by  any 
alternating  system  having  two  wires  per  phase.  That  is,  if  100 
tons  of  copper  are  required  for  a  given  transmission  by  con- 
tinuous current,  single-phase  alternating,  two  phase  with  two 
circuits,  or  three  phase  with  three  circuits,  75  tons  will  suffice 


182  ELECTRIC    TRANSMISSION  OF  POWER. 

for  the  same  transmission  by  the  star  or  mesh  three-phase  cir- 
cuit without  any  increased  loss  of  energy.  The  proof  of  this 
saving  is  very  simple.  Assume  a  three-phase  circuit  carrying 
a  non-inductive  load  at  V  volts  between  lines,  the  current  in 
each  line  being  /  and  the  resistance  r.  Then  for  a  star  con- 
nection, as  we  have  already  seen,  the  voltage  in  each  branch 

to  the  neutral  point  o  (Fig.  90)  is  V  —-=,  the  current  in  each 

A/3 

branch  is  /,  the  power  in  each  branch  is  —  -  IV,   and  the  total 

v  3 
power  is  IV  ^'$. 

The  loss  in  each  branch  of  the  circuit  is  obviously  /V,  and 
the  total  loss  for  the  above  power  3/V.  Now  let  the  same 
amount  of  power  be  transmitted  by  a  single  phase  circuit  at 
the  same  voltage  V.  The  current  will  evidently  have  to  be 
/y^T  Let  r'  be  the  resistance  of  one  of  the  two  monophase 
wires,  such  that  the  total  loss  shall  be  3/V  as  before.  The 
resistance  of  the  complete  circuit  will  be  2r'9  and  the  total  loss 
6/V.  But  since 

6/V  =  3/V, 

r'  =  -- 

2 

That  is,  the  resistance  of  each  of  the  monophase  wires  must  be 
only  one  half  the  resistance  of  a  single  three-phase  wire.  The 
cross  section  of  each  monophase  wire  must  then  be  double  the 
cross  section  of  one  three-phase  wire.  If  the  weight  of  the  lat- 
ter be  w,  the  total  weight  of  the  three-phase  copper  will  be 
3#/,  while  the  weight  of  the  two  monophase  leads  of  double 
cross  section-  will  evidently  be  470  for  a  circuit  of  the  same 
length.  A  mesh  connected  three-phase  system  leads  to  ex- 
actly the  same  result,  since  the  voltage  in  each  branch  is  V 

(see  Fig.  91),  the  current  is  —  —  ,  the  power  per  branch  —   IV, 

V3 


the  total  power  /Fy^,  and  the  loss  3/V,  as  before. 

The  result  seems  so  singular  that  in  the  early  days  of  the 
three-phase  system  it  was  slow  to  be  accepted  by  the  public, 
until  checked  experimentally  with  the  greatest  precision,  and 
by  various  experimenters.  A  similar  saving  can  be  effected 
by  the  use  of  some  other  polyphase  combination  circuits,  but 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       183 

it  happens  that  the  three-phase  combination  is  the  one  least 
open  to  practical  objections. 

In  actual  working  the  two-phase  system  is  nearly  always 
installed  with  a  complete  circuit  per  phase  as  regards  the  dis- 
tribution circuits,  unless  for  short  connections  to  apparatus; 
the  three-phase  system  is  used  with  the  star  or  mesh  combina- 
tion, except  for  occasional  special  work,  and  the  more  compli- 
cated polyphase  systems  are  practically  not  used  at  all. 

In  speaking  of  the  voltage  of  an  alternating  circuit,  it  must 
be  borne  in  mind  that  we  do  not  mean  the  voltage  correspond- 
ing to  the  extreme  crest  of  the  E.  M.  F.  wave,  but  that  volt- 


FIG. 


age  which,  multiplied  by  the  current  in  a  non-inductive  circuit, 
equals  the  energy  in  that  circuit.  This  effective  working  volt- 
age bears  no  fixed  relation  to  the  real  maximum  voltage,  since 
their  ratio  evidently  varies  with  the  shape  of  the  E.  M.  F.  wave. 
For  a  sine  wave  the  ratio  is  1.414,  so  that  an  alternating  working 
pressure  of  1,000  volts  means  a  maximum  voltage  of  1,414.  As 
may  be  judged  from  Fig.  89,  tnis  ratio  is  very  nearly  true  for 
the  best  modern  alternators. 

Save  in  rare  instances  the  work  of  power  transmission  is 
done  by  two-phase  or  three-phase  currents.  Abroad  some  pure 
single-phase  plants  are  in  operation  with  fairly  good  results, 
but  the  difficulty  of  getting  good  single-phase  motors  has  so  far 
rather  checked  development  along  this  line.  In  this  country 
the  "  monocyclic  "  system  has  been  introduced  to  some  extent 
for  simplicity  in  the  connections  for  lighting  purposes,  and  in  a 
few  cases  the  apparatus  is  used  largely  for  motor  purposes. 

In  this  system  there  is  a  main  armature  winding  to  which 
the  lighting  circuits  are  connected  as  in  ordinary  single-phase 
working,  while  a  subsidiary  armature  winding  furnishes  mag- 
netizing current  for  the  motors.  The  general  arrange- 
ment of  the  armature  coils  is  shown  in  Fig.  92.  The 


i86 


ELECTRIC    TRANSMISSION  OF  POWER. 


There  are  two  rings  of  laminated  iron  placed  side  by  side,  and 
connected  by  the  iron  keeper  bars  C.  On  the  interior  face  of 
each  ring  is  a  two-phase  winding  B,  similar  to  that  shown  in 
Fig.  82. 

In  front  of  these  coils  revolves  the  interior  field  magnet,  of 
which  a  pole-piece  is  shown  at  A.  This  magnet  structure  is 
duplex,  like  the  armature,  carrying  two  crowns  of  laminated 


FIG.  95. 

poles.  Between  these  and  supported  by  the  armature  struc- 
ture is  the  field  coil  A,  Fig.  95,  inside  of  which  the  field  magnet 
revolves.  The  relation  of  the  parts  is  shown  in  Fig.  95. 
Obviously  all  the  poles  at  one  end  of  the  machine  are  north 
poles,  those  at  the  other  end  south  poles,  so  that  both  sides 
of  the  armature  are  required  to  complete  the  magnetic  struc- 
ture. As  these  poles  revolve  they  increase  and  decrease  the 
magnetic  induction  through  the  armature  coils  B,  Machines 
of  this  magnetic  character  are  known  as  "  inductor  "  dynamos. 
The  example  shown  has  upon  it  no  moving  wire  whatever,  and 
is  admirably  adapted  for  high  voltage  work. 


PLATE  III. 


TRANSMISSION  BY  ALTERNATING   CURRENTS.      187 

In  the  more  common  form  of  internal  pole  machine,  the 
field  magnet  takes  the  shape  of  a  star  of  outwardly  radiating 
poles  wound  so  that  each  alternate  pole  is  north  and  the  inter- 
mediate poles  south.  All  are  generally  excited  in  series,  the 
current  being  taken  in  via  two  small  slip  rings,  while  the  arma- 
ture currents  are  taken  from  appropriate  stationary  connec- 
tions. The  armature  ring,  having  coils  in  slots  in  its  inner 
face,  is  built  up  in  sections  of  laminated  iron  in  the  ordinary 
way,  with  ample  spaces  for  ventilation.  As,  however,  it  is 
c*ten  of  great  size  and  many  separate  punchings  are  required 
to  go  once  around  the  armature,  a  specially  rigid  exterior 
armature  spider  is  needed  to  support  the  laminated  mass. 
The  coils  are  wound  on  forms,  thoroughly  insulated,  and 
are  then  slipped  into  place  in  the  armature  slots  and  secured 
by  wooden  wedges. 

Plate  III  shows  the  field  and  the  two  halves  of  the  armature 
of  a  modern  high  voltage  polyphase  generator  for  direct  con- 
nection to  the  prime  mover.  In  this  case  the  diameter  of  the 
armature  frame  is  so  great  that  if  has  been  found  desirable  to 
design  it  as  a  hollow  circular  truss  in  order  to  give  it  the  nec- 
essary rigidity  against  distortion  by  its  own  weight  and  by 
inequality  of  magnetic  pull,  if  there  were  a  trifling  eccentricity 
due  to  wear  of  the  bearings.  In  some  of  the  early  machines 
-of  large  diameter  flexure  from  the  weight  alone  was  very 
troublesome.  Half  the  armature  coils  are  shown  in  place  and 
wedged  in,  and  a  coil  belonging  in  the  second  half  is  all  ready 
to  put  in  place.  Four  shapes  of  coils  are  necessary  to  com- 
plete  this  winding,  but  they  can  be  kept  well  clear  of  each 
other  at  the  ends  and  are  easy  to  put  in  and  take  out,  so  that 
in  case  of  damage  a  coil  can  be  easily  replaced,  although  it 
may  sometimes  be  necessary  to  move  several  others  to  get  at 
the  damaged  one.  An  injured  coil,  however,  can  readily  be  put 
out  of  circuit  by  cutting  it  loose  at  the  ends,  insulating  them, 
and  connecting  the  adjacent  coils  of  the  same  phase  across  the 
dead  one.  A  generator  so  temporarily  repaired  in  a  few  min- 
utes can  be  run  until  opportunity  offers  for  permanent  repairs, 
and  can  even  be  worked  in  parallel  with  others  without  mater- 
ial difficulty. 

To  facilitate  repairs  the  armatures  of  large  revolving  pole 
machines  are  often  carried  on  a  sliding  bed,  so  that  they  can 


i88 


ELECTRIC   TRANSMISSION  OF  POWER. 


be  shifted  by  their  own  width  along  the  shaft,  exposing  the 
windings  of  both  armature  and  field. 

The  field  is  really  a  compact,  massive  fly-wheel  with  the 
poles  bolted  on  its  rim,  the  pole  surfaces  being  shaped  so  as  to 
give  as  nearly  as  may  be  a  sinusoidal  wave.  The  pole-pieces 
are  generally  laminated,  at  least  near  the  tips,  and  are  some- 
times provided  with  ventilating  spaces  like  those  in  the  arma- 
ture. 

The  advantage  of  revolving  field  generators  is  so  great  in 
point  of  easy  insulation  and  ready  collection  of  even  very  great 
currents,  that  this  type  of  machine  has  been  rapidly  displac- 
ing the  older  form  for  high  voltage  work,  and  indeed  for  large 


FIG.  96. 

work  of  every  kind.  In  such  generators  voltages  of  10,000 
and  12,000  are  now  quite  common,  and  the  limit  has  not  been 
reached. 

In  large  polyphase  generators  the  question  of  regulating  the 
voltage  in  response  to  changes  of  load  is  a  serious  one,  and  no 
final  solution  of  it  has  as  yet  been  reached.  It  is  not  econom- 
ical to  build  generators  with  so  small  inherent  variation  of  volt- 
age as  is  in  itself  desirable.  In  small  polyphase  machines 
compounding  has  been  accomplished  with  an  arrangement  of 
parts  similar  to  that  shown  in  Fig.  80,  the  connections  being 
So  modified  as  not  to  take  the  commutated  current  from  a 
single  phase.  This  is  troublesome  in  machines  requiring  con- 


TRANSMISSION  BY  ALTERNATING   CURRENTS.       189 


siderable  energy  for  the  field  excitation,  and  besides  it  only 
compounds  correctly  for  a  particular  value  of  the  power  fac- 
tor, which  in  many  plants  is  constantly  changing. 

Several  modern  methods  of  compounding  direct  the  com- 
pounding at  the  exciter.  A  rotary  converter  is  used  as  ex- 
citer, and  the  voltage  at  its  commutator,  which  depends  on  the 
alternating  voltage  applied  at  the  slip-rings,  is  modified  in  va- 
rious ways  in  response  to  changes  in  the  magnitude  and  phase 
of  the  working  currents  from  the  generator.  A  typical  method 
of  this  kind,  successfully  applied  by  the  author  half  a  dozen  years 
ago,  is  shown  diagrammatically  in  Fig.  96.  Here  the  generator 
fields  A  A'  are  fed  from  the  commutator  end  of  a  rotary  con- 
verter FF.  Current  from  the  main  collecting  rings  a  is  led  to 


160, 


150 


140 


J130 
120 
110 


15 


80  K.W. 


FIG.  97. 

the  collecting  rings  b  of  the  exciter  through  the  reactive  coils 
c  c  c  on  the  cores  M  M  M,  which  are  also  wound  with  series 
turns  d  d  d  in  the  main  leads  of  the  generator.  At  light  loads 
the  voltage  at  b  is  cut  down  by  the  reactance,  while  as  the  main 
current  increases  or  lags  the  series  turns  d  d  d  raise  the  vol- 
tage a  b)  and  hence  strengthen  the  generator  field.  By  prop- 
erly proportioning  the  coils  c  c  c,  d  d  d,  and  their  cores 
M  M  M,  the  apparatus  can  be  made  to  regulate  the  voltage 
very  closely  for  all  loads  of  the  generator,  inductive  or  non- 
inductive,  or  even  may  overcompound  on  inductive  load  so  as 
to  compensate  for  the  change  in  the  inductance  of  the  system. 
Fig.  97  shows  the  working  of  this  device  when  arranged  to 
show  extreme  overcompounding  on  inductive  load.  The  gen- 


190  ELECTRIC    TRANSMISSION  OF  POWER. 

erator  chosen  was  one  which  uncompounded  would  drop  its  volt- 
age about  40  per  cent,  on  a  heavy  inductive  load.  Curve  A 
shows  the  regulation  of  the  secondary  voltage  on  non-inductive 
load,  curve  B  the  overcompounding  produced  by  a  load  of 
induction  motors  running  light,  having  a  power  factor  of  not 
over  0.25. 

The  same  general  principle  has  been  lately  applied  in  several 
forms  with  very  promising  results.  An  interesting  modifica- 
tion is  the  compensated  field  alternator  recently  brought  out 
by  the  General  Electric  Company,  and  shown  in  Plate  IV. 
Here  the  exciter  armature  is  on  the  shaft  of  the  main  machine, 
and  is  in  a  field  having  the  same  number  of  poles,  so  that  it 
revolves  synchronously  pole  for  pole  with  its  generator.  Ex- 
citer and  main  fields  are  fed  in  shunt  from  the  exciter  commu- 
tator, but  the  exciter  armature  also  receives  through  its 
collector  rings  an  auxiliary  current  derived  from  series  trans- 
formers in  the  main  leads  of  the  generator.  This  device  holds 
the  voltage  with  beautiful  precision  under  ordinary  changes  of 
load  and  lag,  but  the  necessity  of  being  in  mechanical  synchron- 
ism is  somewhat  embarrassing,  save  in  high  speed  machines. 

Along  such  lines  as  these  good  results  are  certainly  attain- 
able, and  in  addition  there  are  several  automatic  devices  for 
working  a  rheostat  in  the  generator  field  so  as  to  hold  the  volt- 
age constant,  irrespective  of  load  or  lag.  These  perform 
fairly  well,  but  are  apt  to  require  attention  to  the  working 
parts.  They  do  best  when  dealing  with  rather  slow  variations 
in  load,  and.  while  they  need  rather  careful  handling,  are  capa- 
ble of  giving  very  good  regulation.  The  whole  matter  is  at 
present  in  a  transitional  stage,  and  practice  has  not  yet  settled 
into  any  definite  lines. 

As  a  matter  of  fact,  in  much  power  transmission  work  com- 
pound winding  is  not  necessary,  since  the  machines  hold  their 
voltage  closely  without  it  if  well  designed,  and  in  large  plants 
the  variations  of  load  are  usually  so  gradual  that  the  voltage  at 
the  end  of  the  transmission  line  can  be  easily  kept  constant 
by  hand  regulation.  Again,  in  many  transmission  plants  sev- 
eral lines  are  fed  by  one  generator,  so  that  no  compounding 
wouid  suit  all  the  lines;  and  whenever  a  sub-station  is  in- 
stalled, the  secondary  voltage  has  to  be  kept  constant  by 
special  regulation  in  any  event. 


TRANSMISSION  BY  ALTERNATING   CURRENTS. 


TRANSFORMERS. 

The  alternating  current  transformer  is  merely  a  glorification, 
as  it  were,  of  the  fundamental  idea  shown  in  Fig.  4,  p.  12. 
The  loops  A  and  B  are  expanded  into  massive  coils  and  are 
given  a  very  perfect  magnetic  core  of  laminated  iron,  but  the 
principle  is  unchanged. 

In  Fig.  98,  A  is  a  core  composed  of  soft  iron  plates  perhaps 
•yJT  inch  thick,  stamped  into  the  form  shown,  and  then  built  up 
together  like  the  leaves  of  a  book,  B  is  a  coil  of  insulated  wire 
wound  in  a  spiral  around  one  side  of  the  core,  and  C  is  a  single 
loop  of  heavy  insulated  copper  bar  around  the  other  side. 
Now  suppose  an  E.  M.  F.  is  suddenly  applied  to  the  terminals 


FIG.  98. 

of  the  coil  B,  the  loop  C  being  left  open.  Current  will  flow 
through  B  in  amount  determined  by  its  resistance  and  induct- 
ance, setting  up  a  magnetic  field  throughout  the  mass  of  A. 
If  the  current  is  an  alternating  one  an  alternating  magnetic 
field  will  be  set  up  in  A,  and  the  current  in  B  will  settle  down 
to  that  value  which  is  determined  by  the  resistance  and  induct- 
ance of  the  coil.  The  energy  represented  by  this  current  is 
spent  in  heating  the  coil  and  in  doing  work  by  the  reversal  of 
magnetism  in  the  core  A.  The  current  thus  engaged  lags 
behind  its  E.  M.  F.  as  in  other  cases  of  inductive  circuit,  the 
power  factor  at  no  load  being  in  ordinary  cases  from. 6  to  .7. 
Now  close  the  loop  C.  Current  opposing  the  current  in  B 
will  be  at  once  set  up.  The  magnetizing  effect  of  this  reverse 


192  ELECTRIC   TRANSMISSION  OF  POWER. 

current  opposes  the  magnetization  due  to  B,  and  hence  tends 
to  cut  down  the  inductance  imposed  on  B,  which  is,  as  we  have 
already  seen,  determined  by  the  magnetic  induction  through 
its  core.  To  this  action  B  simultaneously  responds  with  an 
increased  current,  so  that  any  increase  of  the  current  in  C  and 
its  consequent  demagnetizing  action,  is  automatically  compen- 
sated by  an  increased  current  in  B.  The  increase  of  energy 
represented  by  this  compensates  for  the  energy  due  to  the 
current  in  C.  Energy  is  thus  virtually  transferred  from  the 
primary  circuit  B  to  the  secondary  circuit  C. 

Now  as  to  the  voltage  of  these  two  circuits.  The  energy 
in  the  two  circuits  is  evidently  equal  save  for  losses  in  the  iron 
and  copper,  which  amount  ordinarily  to  only  a  few  per  cent. 

For  any  given  magnetization  in  A  the  inductive  E.  M.  F. 
in  B  is  proportional  to  the  total  number  of  turns  in  the  coil; 
so  also  the  induced  E.  M.  F.  in  the  secondary  is  proportional 
to  the  number  of  turns  in  it.  That  is  for  a  certain  rate  of 
change  of  the  magnetic  induction  in  A,  the  induced  E.  M.  F. 
is  the  same  per  turn  throughout  A,  whether  that  E.  M.  F. 
appears  as  inductance  in  B  or  secondary  E.  M.  F.  in  C. 
Hence  the  E.  M.  Fs.  across  the  terminals  of  the  primary  and 
secondary  coils  are  proportional  to  the  respective  numbers 
of  turns  in  those  coils.  But  the  energy  in  the  two  is  sub- 
stantially equal,  and  hence  the  currents  in  primary  and 
secondary  must  be  inversely  proportional  to  the  respective 
E.  M.  Fs.  In  Fig.  98  are  shown  seven  primary  turns  and 
one  secondary  turn.  Therefore  the  secondary  E.  M.  F.  is 
one-seventh  the  primary  E.  M.  F.,  and  the  primary  current 
is  one-seventh  the  secondary  current.  For  the  same  density 
of  current  in  amperes  per  square  inch  the  secondary  turn 
must  have  seven  times  the  cross-section  of  the  primary  con- 
ductor. By  simply  changing  the  relative  number  of  primary 
and  secondary  turns, — the  ratio  of  transformation, — electrical 
energy  at  any  voltage  can  be  transformed  to  any  other  voltage 
with  trifling  loss  if  the  apparatus  be  properly  constituted. 

The  losses  which  exist  are  of  three  kinds.  First  is  the  loss 
due  to  the  resistance  of  the  copper.  This  at  light  loads  is 
very  trifling,  but  increases  with  the  square  of  the  load,  being 
numerically  equal  in  watts  to  C13./?,  as  in  all  cases  of  loss 
through  resistance. 


TRANSMISSION  BY  ALTERNATING   CURRENTS.      193 

Second  comes  the  loss  through  hysteresis — virtually  mag- 
netic friction — produced  by  the  alternate  reversals  of  mag- 
netization in  the  iron  core.  This  is  nearly  constant  at  all 
loads  and  is  kept  as  low  as  possible  by  securing  the  best 
possible  iron,  and  working  it  at  rather  low  magnetization, 
since  the  hysteretic  loss  increases  very  rapidly  as  the  iron  is 
more  and  more  strongly  magnetized. 

Finally  comes  the  loss  from  eddy  currents  in  the  core. 
This  is  due  to  the  fact  that  the  core  is  a  fairly  good  conductor, 
and  currents  are  induced  in  it  for  precisely  the  same  reason 
that  they  are  induced  in  the  secondary  winding.  These  eddy 
currents  are  largely  reduced  by  carefully  laminating  the  core 
across  the  natural  direction  of  flow  of  these  currents,  and 
insulating  the  laminae  with  sheets  of  tissue  paper  or  with 
varnish.  The  loss  from  eddy  currents  is,  generally  speaking, 
of  about  the  same  magnitude  as  the  hysteretic  loss,  and  in 
transformer  practice  the  two  are  usually  lumped  together  and 
denominated  core  loss. 

By  careful  construction  and  design  these  losses  can  be  kept 
very  small  compared  with  the  total  output.  The  following 
data  from  a  test  of  a  7,500  watt  transformer  designed  for  a 
frequency  of  15,000  to  16,000  alternations  per  minute,  about 
125  to  135 ~,  will  give  a  clear  idea  of  the  results  that  can  be 
reached  commercially: 

Output,      .       :.        .:..:...        .        .  .     7.5  KW 

Transformation  ratio,      .          .         .         .....          20 :  I 

Full  load  amperes  (primary),  •     .         ....         .  .  .3.6 

Full  load  amperes  (secondary),         .         ....         .        72.0 

Resistance  (primary)  ohms,          .         .         .         »         .  .6.15 
Resistance  (secondary)  ohms,           .        -.         .      ~.         .  .012 

Total  C2  R  loss  (watts),      -         .      - .        .         .         .  143. 

Total  core  loss  (watts),  ..        *         »         ...        78. 

Primary  current  (no  load),  .  .         .         .  .       .063 

Power  factor  (no  load),  ......  .595 

Total  C  R  drop  (per  cent.),         .'       .  ^       ...         R.  .     1.9 

The  efficiency  curve  of  this  transformer  at  various  loads  is 
given  in  Fig.  99.  The  interesting  feature  of  this  curve  is  the 
very  uniform  efficiency  from  half  load  to  full  load,  with  a  maxi- 
mum of  97.4  per  cent,  at  three-quarters  load.  This  is  the 
result  of  a  relatively  very  small  core  loss.  Even  at  one-tenth 


194 


ELECTRIC   TRANSMISSION   OF  POWER. 


the  normal  load  the  efficiency  is  still  good,  over  90  per  cent.^ 
although  the  curve  falls  more  rapidly  below  half  load. 

Still  larger  transformers,  such  as  are  used  for  heavy  power 
transmission  work,  are  even  more  efficient  than  the  one  here 
described,  although  the  room  for  increase  is  very  limited 
indeed.  Within  the  last  few  years  the  improvement  in  com- 
mercial transformers  has  been  very  great.  In  practice  they 


PER  CENT  EFFICIENCY 

JB  g?  g 

x^ 

.«—  -  • 

/ 

j_ 

>          i          2          ; 

J              4               5              o              7              8 
OUTPUT  IN  K.W. 

IMG.  99. 

are  seldom  so  simple  in  form  as  in  Fig.  98,  the  core  plates 
being  universally  built  up  of  several  pieces,  so  that  the  coils 


FIG.  100. 


may  be  wound  in  forms  .and  slipped  into  their  respective  places 
on  the  core.  One  of  the  forms  which  has  been  widely  used 
is  shown  removed  from  its  case  in  Fig.  100.  The  hollow 
rectangle  A  forms  the  main  part  of  the  core,  while  the  bridge 
piece,  B,  is  built  up  separately  as  the  core  of  the  coils, 
together  with  which  it  is  forced  into  the  position  shown. 
The  secondary  coil  immediately  surrounds  the  bridge,  and 


7'KANSMISSION  BY  ALTERNATING   CURRENTS.      195 

outside  of  it  is  the  primary  coil.  Both  coils  are  of  course 
elaborately  insulated.  Another  familiar  form  of  transformer 
is  shown  in  Figs.  101  and  102.  Here  the  core,  Fig.  101,  is 
built  up  of  straight  rectangular  slips  of  iron  into  a  hollow  rect- 
angle upon  the  longer  sides  of  which  the  coils  are  wound.  The 
whole  assembled  core  and  coils  are  shown  in  longitudinal 


FIG.  101. 

section  in  Fig.  102.  This  form  of  construction  gives  the 
coils  a  large  available  cooling  surface  and  simplifies  their 
insulation  somewhat,  although  magnetically  the  arrangement 
of  Fig.  100  is  to  be  preferred. 

As  transformers  are  usually  inclosed  in  tight  iron  boxes  to 
protect  them  from  the  weather,  the  heat  generated  in  the  coils 
and  core  has  a  rather  poor  chance  to  escape,  and  the  tempera- 


FIG.  i-o2. 

ture  may  therefore  rise  higher  than  is  safe  for  the  insulation. 
It  is  usual  to  take  special  precautions  to  prevent  this  over- 
heating. One  of  the  commonest  and  best  devices  for  this 
purpose  is  the  subdivision  of  the  core  into  bunches  of  laminae 
separated  by  air  spaces. 

This  arrangement  is  well  shown  in  Fig.  103,  in  which  the 
core  is  provided  with  a  dozen  of  these  ventilating  spaces. 
The  arrangement  of  the  coils  is  somewhat  like  that  of  Fig.  100. 


196 


ELECTRIC    l^RANSMISSION   OF  POWER. 


As  an  additional  precaution  against  overheating,  the  trans- 
former case  is  often  filled  with  heavy  mineral  oil  after  the 
core  is  in  place.  This  both  provides  additional  insulation, 
and  facilitates  the  transfer  of  heat  from  the  core  and  coils  to 
the  iron  case,  whence  it  is  radiated  to  the  surrounding  air.  In 
very  large  transformers  the  primary  and  secondary  windings 
are  often  built  up  of  thin  flat  sections  assembled  with  spaces 
between  them. 

For   huge   transformers  such   as   are   used  for   sub-station 
work,  means  are  generally  provided  for  artificial  cooling.    Two 


FJG.   103. 

methods  are  at  present  in  use  for  this  purpose.  One  is  the 
use  of  a  blast  of  air  from  a  small  blower  streaming  through 
the  interstices  provided  in  core  and  coils,  and  rapidly  carrying 
away  the  heat  generated.  The  other  is  applied  to  oil-filled 
transformers,  and  consists  in  cooling  the  oil  by  a  worm  in 
the  transformer  case  through  which  cold  water  is  allowed 
to  flow,  or  with  a  small  pump  circulating  the  oil  itself  slowly 
through  a  worm  cooled  by  water.  Either  plan  is  very 
effective,  and  both  are  extensively  used. 

With  properly  designed  transformers  there  is  no  difficulty 
in  dealing  with  any  voltage  now  in  use,  without  the  device 
of  connecting  transformers  in  series,  which  was  formerly  often 
employed  for  high  voltage.  If  transformers  are  of  similar 
size  and  design,  they  can  be  run  in  parallel  with  the  utmost 
facility,  and  may  very  often  be  thus  "banked"  most  advan- 


TRANSMISSION  BY  ALTERNATING   CURRENTS.      197 

tageously,  as  with  such  connection  it  is  easy  to  proportion 
the  number  of  transformers  in  use  to  the  load,  so  that  they 
•can  be  worked  nearly  at  full  load,  and  consequently  at  their 
best  efficiency. 

In  general  the  larger  the  transformer  the  higher  its 
efficiency,  though  the  improvement  is  very  slow  after  the 
output  reaches  25  KW  or  thereabouts.  The  curve  of  Fig. 
104  shows  the  change  in  full  load  efficiency  with  the  size  of 
transformer  as  found  in  the  best  American  practice. 

The  data  here  given   relate   to   transformers  of   the   kind 


PER  CENT  EFFICIENC1/ 

8  %  § 

( 

50                     100                    150                    300                   25 

K.W.  OUTPUT 

FIG.  104. 

employed  for  power  transmission  work,  as  now  produced  by 
the  best  makers.  The  sizes  above  50  KW  are  frequently 
artificially  cooled.  The  frequency  is  taken  at  6o~  to  yo~, 
and  the  figures  do  not  apply  to  transformers  originally 
designed  for  higher  frequencies.  At  lower  frequencies  the 
efficiencies  are  likely  to  be  a  fraction  of  a  per  cent,  lower,  but 
at  any  frequency  within  the  range  of  ordinary  working  a  first- 
class  transformer  of  50  KW  capacity  or  upward  can  be 
depended  on  for  a  full  load  efficiency  of  just  about  98  per 
cent.,  and  a  half  load  efficiency  about  one  per  cent,  lower. 
With  care  in  planning  a  sub-station  equipped  with  these  large 
transformers  the  loss  under  normal  conditions  of  working 
should  not  exceed  2^£  per  cent. 

For  polyphase  work  it  is  the  almost  universal  custom  in  this 
country  to  employ  simply  groups  of  ordinary  standard  trans- 


198 


ELECTRIC   TRANSMISSION  OF  POWER. 


formers.  Abroad,  composite  transformers,  transforming  two 
or  more  phases  in  a  single  structure,  are  often  used.  The 
intent  of  this  arrangement  is  to  utilize  more  fully  the  iron 


FIG.  105. 

core  by  making  it  common   to   the  several  phase  windings. 
Fig.  105   shows  in  rudimentary  form   the  application  of  this 


FIG.  106. 

principle  to  a  three-phase  circuit.  Three  laminated  cores, 
with  the  laminae  running  vertically,  are  united  at  the  ends  by 
laminated  yokes  somewhat  in  the  manner  shown.  Each  core 
receives  the  primary  and  secondary  windings  belonging  to  a 
single  phase,  while  the  iron  belongs  to  the  three  in  common. 


TRANSMISSION  BY  ALTERNATING  CURRENTS. 


199 


The  arrangement  is  akin  to  the  mesh  connection  of  three- 
phase  circuits.  Fig.  106  shows  the  form  actually  taken  by 
a  small  three  phase  transformer  built  up  in  this  manner.  It 
is  a  very  neat  and  compact  structure  and  certainly  convenient 
for  small  work.  In  this  country  it  is  found  that  with  American 
costs  of  labor,  the  saving  due  to  a  common  use  of  core  iron 
is  more  than  counterbalanced  by  the  extra  work  of  building 
up  the  composite  structure,  besides  which  the  distribution  of 
the  iron  in  three  cores  is  somewhat  less  advantageous  in  itself 
than  the  concentration  of  each  core  about  its  own  coils. 


PRIMARY     MAINS 


M     PHASE     B 


TRANSFORMER  A       TRANSFORMER 


\ 

I  r 

\ 

L 

]  r 

1        fV 

L 

J 

L 

J 

PHASE     ]A      [ 

SECONDARY   MAINS 

PHASE     B 


FIG.  107. 

Several  arrangements  of  transformers  are  employed  in  poly- 
phase working  corresponding  to  the  various  arrangements  of 
polyphase  circuits.  For  example,  in  two-phase  systems  the 
transformers  are  generally  connected  as  shown  in  Fig.  107. 
This  is  simply  one  transformer  per  phase  connected  in  the  ordi- 
nary manner.  The  two  phases  are  kept  distinct  both  as  regards 
primary  and  secondary  sides  of  the  circuit.  Fig.  108  shows 
the  composite  circuit  method  of  connection.  Both  primary 
and  secondary  circuits  have  one  wire  common  to  both  phases. 
In  this  case  there  is  between  the  outside  wires  of  the  system 
a  higher  voltage  than  exists  between  either  outside  wire  and 
the  common  wire.  This  voltage  is  of  course  the  geometrical 


2OO 


ELECTRIC  TRANSMISSION  OF  POWER. 


sum  of  two  separate  phase-voltages.  As  these  are  pop 
apart  the  resultant  voltage  is  \J 2  times  either  component. 
Not  infrequently  the  primary  arrangement  of  Fig.  107  is  com- 
bined with  the  secondary  circuit  of  Fig.  108.  This  is  the  ordi- 
nary connection  of  two-phase  motors,  which  are  often  built  for 
this  three-wire  circuit.  As  a  rule  all  lighting  connections  and 
all  long  circuits  of  any  kind  are  made  as  shown  in  Fig.  107. 

Transformers  for  three-phase  circuits,  are,  like  the  circuits 
themselves,  very  seldom  worked  with  the  phases  separated, 
but  in  nearly  every  case  are  combined  in  the  star  or  mesh 
connection.  The  former  is  useful  in  dealing  with  very  high 
voltages,  since  the  individual  transformers  do  not  have  to  carry 


II 
/\AAAAA/^WW\AA 


FIG.  108. 

the  full  voltage  between  lines.  Fig.  109  shows  a  diagram  of 
the  star  connection  and  Fig.  no  the  corresponding  mesh.  In 
each  a,  b,  c,  are  the  primary  leads,  and  A,  B,  C  the  correspond- 
ing secondary  leads.  Of  the  two  connections  the  mesh  is 
rather  the  more  in  use  except  for  high  voltage  work,  and  for 
secondary  distribution  with  a  connection  to  the  common 
junction  of  the  transformer  system,  which  connection  has  for 
certain  purposes  very  great  advantages. 

Whether  the  star  or  the  mesh  connection  is  employed,  one 
transformer  per  phase  is  required,  and  this  condition  is  some- 
times inconvenient  as  rendering  necessary  the  use  of  three 
small  transformers  where  a  two-phase  system  would  need  but 
two.  To  obviate  this  difficulty,  what  may  be  called  the 
"  resultant  mesh  "  connection  is  extensively  used,  particularly 
for  motors.  The  principles  on  which  this  is  based  have  already 
been  set  forth. 

Briefly,  if  one  takes  the  geometrical  sum  of  two  E.  M.  Fs. 
not  in  phase  with  each  other,  the  resultant  will  be  less  than  the 


TRANSMISSION  BY  ALTERNATING   CURRENTS.      201 

arithmetical  sum  of  the  components,  and  not  in  phase  with 
either.  From  the  examples  of  geometrical  summation  already 
discussed,  it  is  evident  that  by  varying  the  magnitudes  of  the 
components  and  the  angle  between  them,  /.  e.,  their  phase 


difference,  the  resultant  may  have  any  desired  value  and  any 
direction  with  reference  to  either  component. 

The    "  resultant   mesh"  three-phase   connection  is  shown 


FIG.  no. 

in  Fig.  in.  It  is  composed  of  two  transformers  instead  of 
three  as  in  Fig.  no,  the  E.  M.  F.  between  the  points  A  and  C 
being  the  resultant  derived  from  the  two  existing  secondaries. 
Each  of  these  secondaries  contributes  its  part  of  the  output 
in  the  resultant  phase,  and  the  secondary  circuit  behaves 


2O2 


ELECTRIC    TRANSMISSION  OF  POWER. 


substantially  as  if  it  were  derived  from  the  ordinary  mesh 
connection.  This  arrangement  is  very  convenient  in  motor 
work,  since  it  is  very  simple  and  allows  the  use  of  two  trans- 
formers when  desirable  for  the  required  output.  Sometimes 
a  motor  is  of  a  size  that  is  fitted  better  by  three  standard 
transformers  than  by  two,  or  the  reverse,  and  with  the  choice 
of  the  two  mesh  connections  it  is  often  possible  to  avoid  some 
extra  expense  or  to  utilize  transformers  that  are  on  hand. 

A  very  beautiful  application  of  this  principle  of  resultant 
E.  M.  F.  is  the  change  of  a  two-phase  system  into  a  three- 
phase,  or  vice  versa.  The  method  of  doing  this  is  shown  in 


3 


FIG.  in. 


Fig.  IT*.  Suppose  we  have  two  equal  E.  M.  Fs.  90°  apart,  as 
in  the  ordinary  two-phase  system,  as  the  primary  circuit.  The 
secondary  E.  M.  Fs.  will  still  be  90°  apart,  but  can  be  of  any 
magnitude  we  please.  Let  one  of  these  secondaries  A  C  give 
say  100  volts,  and  tap  it  in  the  middle  so  that  the  halves,  A  D 
and  D  Cwill  each  be  50  volts;  now  wind  the  other  secondary, 
B  D,tor  50  \/ 3  volts,  and  connect  one  end  of  it  to  the  middle 
point  of  the  first  secondary.  Taking  now  the  geometrical 
sums  of  B  D  with  the  two  halves  of  A  C,  the  resultants  are 
equal  to  each  other  and  to  A  C,  and  leads  connected  to  A,  B, 
and  Cwill  give  three  equal  E.  M.  Fs.  120°  apart,  forming  a 
three-phase  mesh  with  two  resultant  E.  M.  Fs.  instead  of  one, 
as  in  Fig.  1 1 1.  The  actual  connection  of  a  1,000  volt  two-phase 
system  to  form  a  loo-volt  three-phase  secondary  system  is 


TRA  NSM1SSION  BY  AL  TERN  A  TING   CURRENTS.      203 

shown  in  Fig.  113.  Reversing  the  operation  by  supplying 
three-phase  current  to  the  three-phase  side  of  the  system 
gives  a  resultant  two-phase  circuit. 


B 


FIG.  112. 


This  change-over  process  is  valuable  in  that  it  allows  a 
three-phase  transmission  circuit  to  be  used  for  the  saving  in 
copper  characteristic  of  it,  in  connection  with  two-phase 
generating  and  distributing  plants,  and  permits  two-phase  and 


FIG.  113. 

three-phase  apparatus  to  be  used  interchangeably  on  the  same 
circuit,  which  is  sometimes  advantageous.  A  somewhat 
analogous  arrangement  permits  the  transformation  of  a 
monocyclic  primary  circuit  into  a  three-phase  or  two-phase 
secondary  form,  as  may  be  convenient,  and  in  fact  any  system 
with  two  or  more  phases  may  be  transformed  into  any  other 
similar  system  in  the  general  manner  described. 


204  ELECTRIC   TRANSMISSION  OF  POWER. 

It  is  worth  noting  that  the  three  phase-two-phase  trans- 
formation shown  in  Fig.  113  can  in  an  emergency  be  very 
readily  made  without  special  transformers  if  one  has  avail- 
able transformers  of  ratios  9  :  i  and  10  :  i,  respectively,  both, 
these  being  obtainable  commercially.  For  the  latter  tapped 
from  the  middle  of  the  secondary,  as  is  common  for  three- 
wire  work,  gives  the  left-hand  half  of  Fig.  113,  while  the  9  :  i 
transformer  is  sufficiently  near  the  required  ratio  to  give  the 
rest  of  the  combination.  Such  an  extemporized  arrangement 
is  very  serviceable  in  operating  three-phase  induction  motors- 
from  two-phase  mains  or  vice  versa,  and  can  be  put  together 
very  easily.  In  default  of  this  it  is  easy  enough  in  using 
standard  transformers  of  makes  in  which  the  secondary  wind- 
ings are  fairly  accessible  to  tap  the  secondary  winding  so  as 
to  leave  about  12  per  cent,  of  it  dead-ended,  and  this  forms 
the  supplementary  transformer  required. 

The  electrician  will  do  well  to  familiarize  himself  with  the 
handling  of  transformers  in  all  sorts  of  connections,  for  in  a 
sudden  emergency  a  little  deftness  in  this  respect  will  often 
extricate  him  from  an  uncomfortable  corner.  For  instance,, 
one  can  connect  transformers  backwards  to  get  high  voltage 
for  testing,  or  with  the  usual  three-wire  secondaries  trans- 
form twice  and  reach  half  the  primary  voltage,  or  put  several 
secondaries  in  series,  with  the  corresponding  primaries  in  mul- 
tiple, or  do  many  other  things  occasionally  useful.  The  chief 
things  to  be  borne  in  mind  are  that  the  normal  currents  in 
primaries  and  secondaries  must  not  be  exceeded,  that  the 
polarities  must  be  kept  straight  and  great  care  exercised  not 
to  inadvertently  get  any  coils  on  short  circuit. 

One  of  the  most  useful  temporary  expedients  is  boosting  the 
primary  voltage  by  means  of  a  standard  transformer  to  meet 
excessive  drop  in  a  long  feeder.  The  process  is  exceedingly 
simple,  being  merely  the  connection  of  the  secondary  in  series 
with  the  line  to  be  boosted,  while  the  primary  is  put  across  the 
mains  as  usual.  The  result  is  that  the  feeder  voltage  is  raised 
by  nearly  the  amount  of  the  secondary  voltage.  Fig.  114 
shows  a  convenient  way  of  arranging  the  connections,  in  which 
one  of  the  primary  lines  is  so  connected  to  a  double  throw 
single-pole  switch  that  while  boosting  goes  on  with  the  switch 
in  the  position  shown,  on  throwing  the  switch  to  the  reverse 


TRANSMISSION  BY  ALTERNATING   CURRENTS.      205. 

position  the  booster  is  cut  out  and  the  line  receives  its  current 
as  usual.  It  must  be  remembered  in  such  boosting  that  the 
strain  on  the  transformer  insulation  is  more  severe  than  usual, 
and  in  particular  that  the  strain  between  secondary  coils  and 
core  is  the  full  primary  voltage,  for  which  provision  is  seldom 
made  in  insulating  secondaries  from  cores.  Hence  in  rigging 
a  booster  transformer  one  of  the  oil-insulated  type  should  be 
chosen,  and  it  should  be  very  carefully  insulated  from  the 
ground.  For  the  same  reason  the  boosting  transformer 
should  be  of  ample  capacity,  so  that  it  will  not  be  likely  to 


FIG.  114. 

overheat,  and  should  in  general  be  treated  rather  gingerly,  like 
any  other  piece  of  apparatus  subject  to  unusual  conditions. 
Nevertheless,  it  is  capable  of  most  effective  service  if  properly 
operated. 

All  these  systems  which  involve  resultant  E.  M.  Fs.  are 
open  to  certain  practical  objections  which  may  or  may  not  be 
important  according  to  circumstances. 

In  the  first  place,  the  resultant  E.  M.  F.  is  less  than  the 
sum  of  the  E.  M.  Fs.  for  which  the  transformers  in  the  com- 
ponent circuits  are  wound.  For  instance,  in  Figs,  in  and  113, 
100  resultant  volts  are  derived  from  transformers  aggregating 
respectively  200  and  186.7  volts,  through  the  secondaries  of 
which  the  resultant  current  has  to  flow.  In  the  former  case 
one-third  and  in  the  latter  case  two-thirds  of  the  total  current 
is  thus  derived  at  a  disadvantage,  using  up  more  transformer 
capacity  for  a  given  amount  of  energy  than  if  the  transformers 


206 


ELECTRIC   TRANSMISSION   OF  POWER. 


were  used  in  the  normal  manner.  On  a  small  scale  the  dis- 
advantage is  seldom  felt,  but  in  heavy  transmission  work  with 
large  transformers  it  may  be  quite  perceptible. 

Second,  the  disturbance  of  any  one  component  voltage  from 
drop  or  inductance,  or  any  shifting  of  phase  between  the  com- 
ponents from  unequal  lag,  disturbs  all  the  resultant  E.  M.  Fs. 
This,  again,  may  or  may  not  be  of  importance,  but  it  must 
always  be  borne  in  mind,  as  in  every  case  of  combined  phases. 

It  is  possible  by  combinations  of  transformers  similar  to 
those  described,  to  obtain  at  some  sacrifice  in  transformer 
capacity  a  single-phase  resultant  E.  M.  F.  from  polyphase 


JIMPLE  SECTION  OF  TRA'NSFORMEfF 

B 

NAAAA/WWW 


3MPOUND  SECTIONlOF  TRANSFORM 
I/WWW\AA/W\MA/VWW 


FIG.  115. 

components,  or  to  split  up  a  single-phase  current,  by  the  aid  of 
inductance  and  capacity,  into  polyphase  currents.  Neither 
process  is  employed  commercially  as  yet,  since  both  encounter 
in  aggravated  form  the  difficulties  common  to  resultant  phase 
working  mentioned  above,  and  others  due  to  the  special  form 
of  the  combinations  attempted.  Combining  polyphase  cur- 
rents for  a  single-phase  resultant  is  a  process  that  would  be 
very  seldom  useful,  but  the  reverse  process  if  it  were  success- 
fully  carried  out  might  be  of  very  great  importance  in  certain 
distribution  problems,  and  especially  in  electric  railway  prac- 
tice, although  in  working  on  the  large  scale  that  offers  the  best 
field  for  alternating  motors  the  disadvantage  of  two  trolleys  is 
at  a  minimum.  One  very  ingenious  method  of  splitting  an  al- 


TRANSMISSION  BY  ALTERNATING    CURRENTS.      207 

ternating  current  into  three-phase  components  is  the  following, 
due  to  Mr.  C.  S.  Bradley,  one  of  the  pioneers  in  polyphase 
work.  His  process  is  essentially  twofold,  first  splitting  the 
original  current  into  a  pair  of  components  in  quadrature 
and  then  combining  these  somewhat  as  in  Fig.  113.  The  appa- 
ratus is  shown  in  diagram  in  Fig.  115.  Here  A  is  the  gen- 
erator, B  the  simple  primary  of  one  transformer  element,  D  a 
condenser,  n  and  /the  sections  of  the  compound  transformer 
primary,  and  g,  /i,  t\  /,  k  the  secondary  transformer  sec- 
tions. The  condenser  D  is  so  proportioned  that  acting  in 
conjunction  with  the  compound  primary  n  I  the  original  cur- 
rent is  split  into  two  components  in  quadrature,  in  B  and  n  I 
respectively.  Then  the  secondaries  are  so  interconnected  as 
to  produce  three-phase  resultant  currents  which  are  fed  to  the 
motor  M.  The  coil  /  gives  one  phase,  the  resultant  of  g  and  k 
another,  and  the  resultant  of  h  and  /  the  third.  The  combi- 
nation of  these  resultants  gives  a  more  uniform  and  stable 
phase  relation  under  varying  loads  than  would  be  obtained 
from  two-phase  secondaries  fed  by  B  and  n  I  respectively. 
The  condenser  is  necessary  in  getting  a  correct  two-phase  re- 
lation in  the  primaries  to  start  with,  and  even  so  the  E.  M.  Fs. 
will  not  stay  in  quadrature  under  a  varying  load  on  the  sec- 
ondaries unless  the  condenser  capacity  be  varied,  but  the  re- 
combination in  the  secondaries  partially  obviates  this  difficulty. 
A  device  brought  out  abroad  by  M.  Korda  for  a  similar  purpose 
omits  the  condenser  and  splits  the  monophase  current  into  two 
components  60°  apart  by  variation  of  inductance  alone,  and 
these  are  utilized  to  give  three-phase  resultants.  The  phase 
relations  thus  obtained  are,  however,  somewhat  unstable,  as 
must  always  be  the  case  in  phase  splitting  by  inductance  alone. 
For  the  energy  supplied  by  a  monophase  current  is  essentially 
discontinuous,  while  the  energy  of  a  polyphase  circuit  has  no 
periodic  zero  values,  so  that  in  passing  from  one  to  the  other 
there  should  be  storage  of  energy  during  part  of  each  cycle  such 
as  is  obtained  by  the  condenser  of  Fig.  115. 


CHAPTER  VI. 

SYNCHRONOUS  AND  INDUCTION  MOTORS. 

THE  principles  of  the  synchronous  alternating  motor  are  a 
snare  for  the  unwary  student  of  alternating  current  working, 
since  they  involve,  when  discussed  in  the  usual  way,  rather  com- 
plicated mathematical  considerations.  And  the  worst  of  it  is- 
that  the  generalized  treatment  of  the  subject  often  causes  one 
to  lose  sight  of  the  fundamental  ideas  that  are  at  the  root  of 
alternating  and  continuous  current  motors  alike.  The  sub- 
ject  is  at  best  not  very  simple,  and  unless  we  are  prepared  to 
attack  the  general  theory  with  all  its  many  considerations,  it  is 
desirable  not  to  cut  loose  from  the  common  basis  of  all  motor 
work. 

Recurring  to  the  rudimentary  facts  set  forth  in  Chapter  I,  we 
see  that  an  electric  motor  consists  essentially  of  two  working 
parts — a  magnetic  field  and  a  movable  wire  carrying  an  elec- 
tric current.  The  motive-power — torque — is  due  to  the  reac- 
tion between  the  magnetic  stresses  set  up  by  the  current  and 
those  due  to  the  field.  The  refinements  of  motor  design  are 
concerned  with  the  efficient  production  of  these  two  sets  of 
stresses  and  their  co-ordination  in  such  wise  that  their  reac- 
tion shall  produce  a  powerful  torque  in  a  uniform  direction.  . 

In  continuous  current  motors,  for  example,  the  field  mag- 
nets are  energized  by  a  part  or  the  whole  of  the  working- 
current,  and  this  current  is  passed,  before  entering  the 
armature,  through  a  commutator  like  that  of  the  generator,  so 
that  in  the  armature  the  direction  of  the  currents  through  the 
working  conductors  shall  be  reversed  at  the  proper  time,  so  as 
to  react  in  a  uniform  direction  with  field  poles  which  are  con- 
secutively of  opposite  polarity.  Were  it  not  for  the  commu- 
tator the  armature  would,  on  turning  on  the  current,  stick  fast 
in  one  position,  as  may  happen  when  there  is  a  defect  in  the 
winding. 

Now,  since  the  function  of  the  commutator  in  the  generator 
is  to  change  a  current  normally  alternating,  so  that  it  shall 

208' ' 


SYNCHRONOUS  AND   INDUCTION   MOTORS.  209 

flow  continuously  in  one  direction,  and  since  the  object  of  the 
commutator  in  the  motor  is  periodically  to  reverse  this  current 
in  the  armature  coils,  thus  getting  back  to  the  original  current 
again,  one  naturally  asks  the  reason  for  going  to  all  this 
trouble.  Why  not  let  the  generator  armature  do  the  revers- 
ing instead  of  providing  two  commutators — the  second  to  undo 
the  work  of  the  first? 

The  reason  is  not  far  to  seek.  In  a  generator  running  at 
uniform  speed  the  reversals  of  current  take  place  at  certain 
fixed  times — whenever  an  armature  coil  passes  from  pole  to  pole, 
quite  irrespective  of  the  needs  of  the  motor.  The  commuta- 
tor on  the  other  hand  reverses  the  current  in  the  motor  arma- 
ture coils  in  certain  fixed  positions  with  respect  to  the  field 
poles  so  as  to  produce  a  continuous  pull,  irrespective  of  what 
the  generator  is  doing. 

If  we  abolish  the  commutators  the  motor  will  run  properly 
only  when  the  alternating  impulses  received  from  the  genera- 
tor catch  the  armature  coils  systematically  in  the  same 
positions  in  which  reversal  would  be  accomplished  by  the  com- 
mutator Hence  for  a  fixed  speed  of  the  generator  the  im- 
pulses will  be  properly  timed  only  when  the  motor  armature 
is  turning  at  such  a  speed  that  each  coil  passes  its  proper 
reversal  point  simultaneously  with  each  reversal  of  the 
generator  current.  If  generator  and  motor  have  the  same 
number  of  poles,  this  condition  will  be  fulfilled  only  when 
they  are  running  at  exactly  the  same  number  of  revolutions 
per  minute.  In  any  case  they  must  run  synchronously  pole 
for  pole,  so  that  if  the  motor  has  twice  as  many  poles  as  the 
generator,  it  will  be  in  synchronism  at  half  the  speed  in  revo- 
lutions per  minute,  and  so  on. 

If  we  try  to  dispense  with  the  commutators  when  starting 
the  motor  from  rest,  the  action  will  obviously  be  as  follows: 
The  first  impulse  from  the  generator  might  be  in  either  direc- 
tion, according  to  the  moment  at  which  the  switch  was  thrown. 
The  reaction  between  this  current  in  the  armature  coils  and 
the  field  poles  might  tend  to  pull  the  armature  in  either  direc- 
tion, but  long  before  the  torque  could  overcome  the  inertia  of 
the  armature  a  reverse  impulse  would  come  from  the  ger.  erator 
and  undo  the  work  of  the  first.  Consequently  the  motor 
would  fail  to  start  at  all. 


210  ELECTRIC    TRANSMISSION  OF  POWER. 

If  the  impulses  from  the  generator  came  very  slowly  indeed, 
so  that  the  first  could  give  the  armature  a  start  before  the 
second  came,  the  armature  would  stand  a  chance  of  getting 
somewhere  near  its  proper  reversal  point  before  the  arrival  of 
the  reverse  current,  and  thus  might  get  a  helping  pull  that 
would  improve  matters  at  the  next  reversal,  but  the  direction 
of  the  first  impulse  would  be  quite  fortuitous.  Starting  the 
armature  in  either  direction  before  the  current  is  thrown  on 
gives  it  a  better  chance  to  go  ahead  if  the  first  impulses  in  the 
wrong  direction  are  not  strong  enough  to  stop  it  altogether. 

We  see,  then,  that  an  alternating  current  derived  directly 
from  the  generator  does  not  give  reversals  in  the  motor  coils 
that  are  equivalent  to  the  action  of  a  commutator,  save  at 
synchronous  speed.  Except  at  this  speed  the  current  from 


ooooo     ~*mw*^     ooooo 

FIG.  116. 

the  generator  does  not  reverse  in  the  motor  armature  coils 
when  the  latter  are  in  the  proper  position. 

Fig.  116  will  give  a  clear  idea  of  the  condition  of  affairs  in 
the  field  and  armature  conductors  of  a  continuous  current 
motor.  Here  6"  and  N  are  the  poles,  and  -f-  and  —  mark  the 
positions  of  the  positive  and  negative  brushes  with  reference 
to  the  armature  winding.  The  solid  black  conductors  carry 
current  flowing  down  into  the  plane  of  the  paper.  The  white 
conductors  carry  current  upward  The  armature  turns  in 
the  direction  of  the  arrow,  and  as  each  conductor  passes  under 
the  brush  the  current  in  it  is  reversed.  This  distribution  of 
current  is  necessary  to  the  proper  operation  of  the  motor,  and 
if  the  brushes  are  moved  the  motor  wi'i  run  more  and  more 
weakly,  and  then  stop  and  begin  to  run  in  the  opposite  direc- 


SYNCHRONOUS  AND  INDUCTION  MOTORS. 


211 


tion,  until  when  the  brushes  have  moved  180°  the  motor  will 
be  running  at  full  power  in  the  reverse  direction.  This  final 
position  means  that  the  currents  in  the  two  halves  of  the 
armature  have  exchanged  directions,  so  that  the  conductors 
originally  attracted  toward  N  and  repelled  from  S,  are  now 
repelled  from  N  and  attracted  toward  S.  If  alternating 
current  from  the  generator  is  led  into  the  windings,  the  dis- 
tribution of  current  shown  in  Fig.  116  must  be  preserved,  and 
since  in  abolishing  the  commutator  the  alternating  current 
leads  are  permanently  connected  to  two  opposite  armature 
coils  through  slip  rings,  the  distribution  of  Fig.  116  can  only 
be  preserved  when  these  leads  change  places  by  making  a  half 
revolution  every  time  the  current  reverses  its  direction. 


FIG.  117. 

Otherwise  the  distribution  of  currents  will  be  changed,  and  the 
motor  will  fail  to  operate,  since  each  reversal  of  current  will 
catch  the  armature  in  a  wrong  position,  and  may  tend  to  turn 
it  in  the  wrong  direction  as  much  as  in  the  right. 

Hence  such  a  motor  must  run  in  synchronism,  or  not  at  all, 
and  to  operate  properly  it  must  either  be  brought  to  full  syn- 
chronous speed  before  the  alternating  current  is  turned  on,  or 
nursed  into  action  by  running  the  generator  very  slowly,  work- 
ing the  motor  into  synchronous  running  at  very  low  speed, 
and  then  gradually  speeding  up  the  generator,  thus  slowly 
pulling  the  motor  up  to  full  speed.  In  practice  the  former 
method  is  uniformly  employed,  and  the  machine  used  as  a 
synchronous  motor  is  substantially  a  duplicate  of  the  alternat- 
ing generator  as  already  described.  In  fact,  it  is  an  alternating 
generator  worked  as  a  motor,  just  as  a  continuous  current 
motor  is  the  same  thing  as  the  corresponding  generator. 

Fig.  117  gives  a  clear  idea  of  the  way  in  which  synchronous 
alternating  motors  are  employed  for  power  transmission. 


212  ELECTRIC  TRANSMISSION  OF  POWER. 

Here  G  is  the  generator  driven  from  the  pulley  P.  S  is  a 
switch  connecting  the  generator  to  the  line  wires  L  L  '.  At  the 
motor  end  of  the  line  is  a  second  switch  S',  which  can  connect 
the  line  either  with  the  synchronous  motor  M,  or  the  starting 
motor  M'.  This  latter  is  usually  some  form  of  self-starting 
alternating  motor  to  which  current  is  first  applied.  M'  then 
gradually  brings  M  up  to  synchronous  speed  ;  when  the  switch 
.S'  is  thrown  over,  the  main  current  is  turned  on  J/,  and  then 
the  load  is  thrown  on  the  driving  pulley  P'  by  a  friction  clutch 
or  some  similar  device. 

Such  a  system  has  certain  very  interesting  and  valuable 
properties.  We  can  perhaps  best  comprehended  them  by 
•comparing  them  with  the  properties  of  continuous  current 
motor  systems. 

In  the  alternating  system  both  generator  and  motor  are 
usually  separately  excited,  which  means  really  that  the  field 
strengths  are  nearly  constant;  as  constant  in  fact  as  those 
in  a  well  designed  shunt-wound  generator  and  motor  for  con- 
tinuous current. 

Now  we  have  seen  that  this  latter  system  is  beautifully  self- 
regulating.  Whatever  the  load  on  the  motor,  the  speed  is 
nearly  constant,  and  the  current  is  closely  proportional  to  the 
load.  If  the  load  increases,  the  speed  falls  off  just  that  minute 
amount  necessary  to  lower  the  counter  E.  M.  F.  enough  to 
let  through  sufficient  current  to  handle  the  new  load.  The 
effective  E.  M.  F.  is  the  difference  between  E,  the  impressed 
E.  M.  F.  and  E'  ,  the  counter  E.  M.  F.  The  current  produced 
by  this  E.  M.  F.  is  determined  by  Ohm's  law. 


where  r  is  the  armature  resistance,  and  since  we  have  seen  that 
the  output  of  the  motor  is  measured  by  the  counter  E.  M.  F., 
IV  =  C  E'  (2) 

where  W,  in  watts,  includes  frictional  and  other  work.  E', 
neglecting  armature  reaction,  is  proportional  to  the  speed  of 
the  armature,  which  falls  under  load  just  enough  to  satisfy 
equation  (2)  by  letting  through  the  necessary  current. 

Now  we  have  seen  that  when  we  abandon  the  commutator  the 
motor  has  to  run  at  true  synchronous  speed,  or  else  lose  its  grip 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


213 


entirely.  How  can  it  adjust  itself  to  changing  conditions  of 
load?  If  the  load  increases,  more  current  is  demanded  to  keep 
up  the  output,  but  the  field  strength  remains  constant,  and  the 
counter  E.  M.  F.  of  the  motor  cannot  fall  by  reduction  of  speed. 
We  must  note  that  while  in  a  continuous-current  motor  the 
counter  E.  M.  F.  of  the  armature  is  constant  at  uniform  speed, 
in  an  alternating  motor  the  counter  E.  M.  F,  varies  like  that  of 
the  generator,  following  approximately  a  sinusoidal  curve,  as  the 
position  of  the  armature  with  respect  to  the  field  poles  varies. 
Hence  at  any  given  instant  the  counter  E.  M.  F.,  the  speed 
and  field  strength  remaining  the  same,  depends  on  the  position 
of  the  motor  armature.  In  Fig.  1 18  we  have  a  pair  of  alternat- 
ing machines,  generator  A  and  motor  B.  In  normal  running 


FIG.  1 1 8. 

at  light  load,  the  two  are  nearly  in  opposite  phase,  since  of 
course  the  impressed  and  counter  E.  M.  Fs.  are  virtually  in 
opposition. 

Now,  if  there  is  an  increase  of  load  the  motor  armature  sags 
backward  a  little  under  the  strain,  thereby  lessening  the  com- 
ponent of  its  counter  E.  M.  F.  that  is  in  opposition  to  the 
impressed  E.  M.  F.  The  current  increases,  and  with  it  the 
torque,  and  the  sagging  process  stops  when  the  torque  is  great 
enough  to  carry  the  new  load  at  synchronous  speed.  The 
change  of  phase  in  the  counter  E.  M.  F.  thus  takes  the  place 
of  change  of  absolute  speed  in  the  continuous  current  motor, 
by  the  same  general  process  of  increasing  the  E.  M.  F.  effec- 
tive in  forcing  current  through  the  circuit.  This  effective 
E.  M.  F.  is  generally  by  no  means  in  phase  with  the  impressed 


214 


ELECTRIC    TRANSMISSION   OF  POWER. 


E.  M.  F.,  and  in  general  the  current  and  the  impressed  E.  M.  F. 
are  not  in  phase  in  a  synchronous  motor.  Here,  as  elsewhere,. 
the  input  of  energy  is 

C  E  cos  cp, 

while  the  output,  which  in  the  continuous  current  motor  is 
simply  the  product  of  the  current  and  the  counter  E.  M.  F., 
in  the  synchronous  motor  depends  evidently  on  such  parts  of 
both  as  are  in  phase  with  each  other,  «.  e., 

W  =  C  E'  cos  qj  (3), 

in  which  cpr  is  the  angle  between  current  and  counter  E.  M.  F. 
Likewise  the  current,  which  in  the  continuous  current  motor 
depends  on  the  effective  E.  M.  F.  and  the  resistance,  now  de- 
pends on  the  counter  E.  M.  F.  and  the  impedance  /.  So  that 


In  this  equation  the  values  of  all  the  quantities  depend  on 
their  relative  directions,  and  by  combining  geometrically  the 


FIG.   119. 

factors  of  (4)  we  can  form  a  clear  idea  of  the  singular  relations 
that  may  be  found  in  synchronous  motor  practice. 

The  construction  is  similar  to  that  found  in   Fig.  51,  p.  130, 
In  Fig.  119,  we  will  start  with  an  assumed  impressed  E.  M.  F. 
of  1000  volts,  a  counter  E.  M.  F.  of  800  volts  and  an  imped- 
ance composed  of  5  ohms  resistance  and  10  ohms  equivalent 
inductance. 

To  begin  with,  we  will  lay  off  the  impressed  E.  M.  F.  A  B, 
and  then  the  counter  E.  M.  F.  B  C,  which  as  we  have  seen  is 
in  partial  opposition  to  A  B.  In  this  case  A  C  is  the  resultant 
E.  M.  F.,  which,  on  the  scale  taken,  is  300  volts.  This,  then,  is 
the  available  E.  M.  F.  taken  up  by  the  inductive  and  ohmic 
drops  in  the  armature.  The  next  step  is  to.  find  C  (eq.  4); 


SYNCHRONOUS  AND  INDUCTION  MOTORS. 


215 


from  /,  and  the  value  of  E — E\  just  obtained.  To  obtain  /,  we 
must  combine  resistance  and  inductance,  as  shown  in  Fig.  109. 
Performing  this  operation,  it  appears  that  /  =  11.18.  Hence 

7OO 

in  the  case  in  hand  C  = 5-  =  26.8+  amperes.    As  to  the 

II.  lo 

direction  of  this  current,  we  know  that  it  is  at  right  angles  to  the 
inductive  E.  M.  F,  /.  e. ,  is  in  phase  with  the  resistance  in  Fig.  120. 
Solving  that  triangle  to  obtain  the  angle  between  the  current 
and  impedance,  it  turns  out  to  be  a  little  over  63°,  being  the 
angle  whose  tangent  is  *£-.  Laying  off  this  angle  a  from  A  Cf 
the  impedance  in  Fig.  119,  we  find  the  current  to  be  in  the 
direction  A  D.  This  current  then  is  out  of  phase  with  the 
impressed  E.  M.  F.  by  the  angle  of  lag  D  A  B.  It  is  also  out 
of  phase  with  the  counter  E.  M.  F.,  though  by  chance  very 


'If 


INDUCTANCE=10 
FIG.  120. 

slightly,  and  lags  behind  the  resultant  E.  M.  F.  A  C,  by  the 
angle  a.  Being  nearly  in  phase  with  the  counter  E.  M.  F.,  the 
gross  output  of  the  motor  is  approximately  26.8  x  800  =  21.4 
KW. 

Now,  what  happens  when  the  load  increases?  The  motor 
armature  sags  back  a  few  degrees  under  the  added  torque,  and 
the  counter  E.  M.  F.  takes  the  new  position  B  C.  The  new 
resultant  E.  M.  F.  is  A  C',  which  on  the  scale  taken  equals  450 


volts.     The  new  value  of  the  current  is  C  = 


.    45° 


ii.  18 


40.25 


amperes,  and  its  phase  direction,  63°  from  A  C',  is  A  D1.  The 
new  angle  of  lag  is  then  D  A  B,  showing  that  under  the  larger 
load  the  power  factor  of  the  motor  has  improved.  If  C  B 
should  lag  still  more,  A  C ',  together  with  the  current,  would 
keep  on  increasing.  Evidently,  too,  the  angle  of  lag  D'  A  B 
will  grow  less  and  less  until  A  C'  B  becomes  a  right  angle, 


216  ELECTRIC    TRANSMISSION  OF  POWER. 

when  in  the  case  shown  it  will  be  very  minute,  and  the  power 
factor  will  be  almost  unity.  Beyond  this  point  the  angle 
€'  A  B  will  obviously  begin  to  decrease,  and  D'  A  B  will  begin 
to  open  out,  again  lowering  the  power  factor  at  very  heavy 
loads. 

Hence  it  appears  that  at  a  given  excitation  there  is  a  par- 
ticular load  for  which  the  power  factor  is  a  maximum,  and  it 
is  evident  from  the  figure  that  in  the  example  taken  this  maxi- 
mum will  be  higher  as  the  inductance  of  the  system  decreases, 
and  also  will  pertain  to  a  smaller  output.  Let  us  now  see 
what  happens  when  the  excitation  of  the  motor  is  varied.  In 
Fig.  121  the  conditions  are  the  same  as  before,  except  that  we 


FIG.  121. 

assume  counter  E.  M.  Fs.  of  500  volts  corresponding  to  C  and 
1,100  volts  corresponding  to  C'.  Examining  the  former,  the 
resultant  E.  M.  F.  is  A  C  =  528  volts,  the  corresponding  cur- 
rent is  47-]-  amperes  and  the  angle  of  lag  D  A  B  is  much 
greater  than  before.  The  power  factor  evidently  would  still 
be  rather  bad  under  increased  loads,  and  worse  yet,  when  at 
lighter  loads  the  angle  A  B  C  decreases.  Lessened  inductance, 
however,  would  help  the  power  factor  by  decreasing  the  angle 
C  A  D,  and  hence  BAD.  Now,  consider  the  result  of 
increasing  the  motor  excitation  to  B  C'  =  1,100  volts.  The 
resultant  E.  M.  F.  now  becomes  A  C',  being  shifted  forward 
nearly  90°,  its  value  is  280  volts  and  the  current  is  25-}- 
amperes.  But  this  current  is  now  in  the  direction  A  D\  a 
being  the  same  as  before,  and  hence  it  no  longer  lags,  but 
leads  the  impressed  E.  M.  F.  by  nearly  45°.  The  power  fac- 
tor is  therefore  still  bad,  but  gets  better  instead  of  worse 
under  loads  greater  than  that  shown.  Inductance  in  the 


SYNCHRONOUS  AXD   INDUCTION  MOTORS.  217 

system  now  improves  the  power  factor,  and  combined  with 
heavy  load  might  bring  the  current  back  into  phase  with  the 
impressed  E.  M.  F 

The  counter  E.  M.  Fs.  corresponding  to  C  and  C'  are 
rather  extreme  cases  for  the  assumed  conditions,  but  it  is  easy 
to  find  a  value  for  the  excitation  which  would  annull  the  lag 
exactly  for  a  particular  value  of  the  load.  Laying  off  in  Fig. 
1 2  T,  C"  A  B  =  C'  AD'  we  find  the  required  counter  E.  M.  F., 
which  is  very  nearly  910  volts.  At  the  particular  output  cor- 
responding to  this  condition,  the  power  factor  is  unity,  the 
current  and  the  impressed  E.  M.  F.  are  in  phase,  and  since 
the  current  is  therefore  a  minimum  for  the  output  in  question, 
the  efficiency  of  the  conducting  system  is  a  maximum.  At 
this  point,  too,  the  energy  is  correctly  measured  by  the  product 
of  volts  and  amperes,  so  that  if  wattmeters  are  not  at  hand 
the  input  at  a  synchronous  motor  can  be  closely  approximated 
at  any  steady  load  by  varying  the  field  until  the  armature  cur- 
rent is  a  minimum,  and  reading  volts  and  amperes. 

Throughout  this  investigation  it  has  been  assumed  that  the 
ratio  of  resistance  and  inductance  has  been  constant.  This  is 
not  accurately  true,  but  is  approximately  so  when  the  induct- 
ance is  fairly  low.  The  phenomenon  of  leading  current  in  a 
synchronous  motor  system  does  not  indicate  that  the  current, 
in  some  mysterious  way,  has  been  forced  ahead  of  the  E.  M.  F. 
which  produces  it,  for  the  impressed  E.  M.  F,  is  not  responsible 
for  the  current,  which  is  determined  solely  by  the  resultant 
E.  M.  F.  behind  which  the  current  invariably  lags. 

The  net  practical  result  of  all  this  is  that  a  synchronous 
alternating  motor,  under  varying  excitation,  is  capable  of 
increasing,  diminishing,  or  annulling  the  inductance  of  the  sys- 
tem with  which  it  is  connected,  or  can  even  produce  the  same 
result  as  a  condenser  in  causing  the  current  to  lead  the  im- 
pressed E.  M.  F.  The  maximum  torque  of  the  motor,  which 
determines  the  maximum  output,  is  determined  by  the  greatest 
possible  value  of  C  E'  cos  qj  consistent  with  the  given 
impedance  and  electromotive  forces.  The  stronger  the  motor 
field,  and  the  less  the  armature  inductances  and  reactions  of 
both  generator  and  motor,  the  greater  the  ultimate  load  that 
can  be  reached  without  overburdening  the  motor  and  pulling 
it  out  of  step. 


218  ELECTRIC   TRANSMISSION  OF  POWER. 

As  regards  the  relation  in  phase  between  current  and  im- 
pressed E.  M.  F.,  the  three  commonest  cases  are  those  for 
which  the  currents  were  computed  for  Figs.  120  and  121.  The 
first,  and  commonly  the  most  desirable,  is  that  in  which  the 
current  lags  slightly  at  small  loads,  gradually  lags  less  and  less, 
comes  into  phase,  or  very  nearly  so,  at  about  average  load,  and 
lags  slightly  again  at  heavy  loads.  The  maximum  efficiency 
of  transmission,  reached  when  the  lag  touches  zero,  is  then  at 
about  average  load.  The  second  and  commoner  case  is  when 
the  motor  is  rather  under-excited,  so  that  the  lag  merely 
reaches  a  rather  large  minimum,  never  touching  zero.  The 
third  case  is  that  in  which  the  current  leads  at  all  moderate 
loads,  passes  through  zero  lag,  and  then  lags  more  and  more. 
The  average  power  factor  may  be  the  same  as  in  the  first  case, 
but  more  energy  is  required  for  excitation,  and  no  advantage 
is  gained  except  in  carrying  extreme  loads,  often  undesirable 
on  account  of  overheating,  or  in  modifying  the  general  lag 
factor. 

It  is  highly  desirable  for  economy  in  transmission  that  the 
product  of  current  and  E.  M.  F.  should  be  a  minimum  for  the 
required  load.  This  condition  can  be  fulfilled  for  the  motor 
circuit  at  any  load  by  changing  the  excitation  until  the  current 
for  that  load  becomes  a  minimum.  Further,  the  field  of  a 
uniformly  loaded  motor  may  in  the  same  way  be  made  to  bring 
the  entire  line  current  of  the  system  to  a  minimum  if  the 
motor  be  o'f  sufficient  capacity.  Thus  a  synchronous  motor 
load  can  be  made  very  useful  in  improving  the  general  condi- 
tions of  transmission.  By  changing  the  motor  excitation  as 
the  load  on  the  motor  or  the  system  varies,  the  power  factor 
can  be  kept  at  or  near  unity  for  all  working  loads. 

Fig.  122  shows  the  power  factor  of  a  synchronous  motor 
somewhat  under-excited,  and  that  of  a  similar  machine  with  a 
field  strong  enough  to  produce  lead  at  moderate  loads.  With 
proper  adjustment  of  its  field,  the  effect  of  a  synchronous 
motor  on  the  general  conditions  of  distribution  is  very  bene- 
ficial. In  curve  A,  Fig.  122,  the  indications  are  that  the  motor 
had  rather  a  high  inductance  and  armature  reaction,  and  the 
excitation  was  decidedly  too  low  for  good  results.  Curve  B 
is  from  a  300  HP  motor,  with  its  field  adjusted  for  zero  lag  at 
about  load.  The  inductance  was  low  and  the  armature 


SYA'CHRONOUS  AND   INDUCTION  MOTORS. 


219 


reaction  small.  The  result  is  somewhat  startling.  Even  at  ^ 
load  the  power  factor  (current  leading)  is  about  .93.  At  half 
load  it  has  passed  .99,  touches  unity,  and  then  slowly  diminishes 
to  very  nearly  .98  (lagging)  at  full  load.  In  this  case  the  gen- 
erator was  held  accurately  at  voltage  while  the  excitation  of 
the  motor  was  uniform.  Both  were  polyphase  machines  wound 
for  2,500  volts. 

When  a  synchronous  motor  is  used  in  this  manner,  it  obvi- 
ously will  show,  at  the  same  load,  values  of  the  current  varying 
if  the  excitation  be  varied.  For  any  load  the  minimum  current 
is  given  by  that  excitation  which  brings  the  current  into  phase 


,  100 


£ 


90 


70 


50 


M.  fc 

PROPORTION  OF   FULL  LOAD 

FlG.    122. 


with  the  impressed  E.  M.  F.  This  point  is  fairly  well  defined. 
At  less  excitation  the  current  lags,  with  more  it  leads. 

Fig.  123  shows  for  a  particular  instance  the  relations  between 
the  current  and  the  excitation  of  the  motor  field,  at  full  load 
of  the  motor.  It  is  evidently  easy  to  adjust  the  excitation  to 
the  proper  point. 

In  the  practical  work  of  power  transmission  the  synchronous 
motor  has  several  salient  advantages  to  commend  it.  At  con- 
stant frequency  it  holds  its  speed  absolutely,  entirely  indepen- 
dent of  both  load  and  voltage  until,  from  excessive  load  or 
greatly  diminished  voltage,  it  falls  out  of  phase  and  stops. 


22O 


ELECTRIC    TRANSMISSION  OF  POWER. 


It  constitutes  a  load  that  is  substantially  non-inductive,  so 
that  it  causes  no  embarrassing  inductive  complications  in  the 
system,  and  takes  current  almost  exactly  in  proportion  to  its 
work. 

Finally  it  can  be  made  to  serve  the  same  end  as  a  condenser 
of  gigantic  capacity  in  compensating  for  inductances  else- 
where in  the  system,  and  thus  raising  the  general  power  factor 
substantially  to  unity. 


70 

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AMPERES  IN  L 
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\ 

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f 

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V-n 

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II 

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9- 

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0                                        10                                       20                                     •  3< 
AMPERES  IN  FIEUD 

FIG.  123. 

As  compensating  disadvantages,  it  must  run  at  one  fixed 
uniform  speed  under  all  conditions,  it  is  not  self-starting,  and 
it  requires  the  constant  use  of  a  continuous-current  exciter. 

For  many  purposes  the  fixed  speed  is  no  objection,  and  in 
most  large  work  the  exciter  can  be  used  without  inconvenience. 
Inability  to  start  unaided,'  even  when  quite  unloaded,  is  on  the 
other  hand  a  very  serious  matter,  and  has  driven  engineers  to 
many  ingenious  subterfuges.  The  simplest  of  these  is  to  pro- 
vide a  starting  motor,  which  is  supplied  with  power  by  any 
convenient  means,  and  serves  to  bring  the  main  machine  up  to 
synchronous  speed.  Then  the  main  current  is  thrown  on,  the 
motor  falls  into  synchronism,  and  the  load  is  taken  up  by 
means  of  a  clutch.  The  difficulty  is  to  start  the  starting 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  *2i 

motor.  In  transmissions  of  moderate  length,  continuous 
current  may  be  delivered  over  the  main  line  from  the  exciter 
of  the  generator  to  the  exciter  of  the  motor,  which  is 
thereby  driven  as  a  motor,  and  brings  the  alternating  motor 
up  to  speed.  As  the  energy  required  for  this  work  is  not 
great,  say  10  per  cent,  of  the  whole  power  transmitted,  it  can 
often  be  delivered  quite  easily.  At  long  distances,  however, 
the  drop  becomes  too  great  for  the  moderate  voltages  avail- 
able with  continuous  current,  and  other  methods  have  to  be 
used. 


FIG.  124. 

The  best  known  of  these  is  one  used  in  a  large  pioneer  alter- 
nating plant  at  Telluride,  Col.  This  method  is  shown  in 
Fig.  124.  The  main  synchronous  motor  *S  is  connected  to  its 
driving  pulley  P  through  the  friction  clutch  C.  On  the  motor 
shaft  is  a  friction  pulley  It.  The  starting  motor  M  is  a  Tesla 
induction  motor,  fitted  to  run  on  a  single-phase  circuit,  and 
supplied  with  a  friction  pulley  Q.  M  slides  upon  ways  so  that 
by  turning  the  handle  //,  Q  can  be  brought  into  contact 
with  R. 

In  starting,  current  is  first  thrown  upon  M,  bringing  it  up  to 
speed.  Then,  the  clutch  C  having  been  opened,  M  is  moved 
until  Q  and  R  are  in  good  contact,  and  rapidly  brings  S  up  to 
speed.  The  current  is  then  thrown  from  M  into  the  armature 
A  of  the  large  motor,  the  load  is  taken  up  by  tightening  the 
clutch  C.  and  M  is  withdrawn  to  its  original  position.  The 
process  is  quite  simple  and  easy,  and  the  apparatus  worked 


222 


ELECTRIC   TRANSMISSION  OF  POWER. 


well,  although  it  has  now  for  several   years  been   replaced  by 
polyphase  machinery. 

Another  method  sometimes  used  is  a  special  commutator  to 
rectify  the  current  applied  to  the  main  motor  armature,  thus 
directing  the  impulses  so  as  to  secure  a  small  starting  torque, 


FIG.  125. 


enough  to  bring  the  motor  to  speed.  Then  the  commutator 
is  abandoned  and  the  motor  falls  to  running  synchronously. 

An  ingenious  modification  of  this  plan  is  found  in  the  self- 
starting  synchronous  motor  of  the  Fort  Wayne  Electric  Cor- 
poration, shown  in  Fig.  125. 

This  machine  has  a  double-wound  armature.  The  main 
winding  is  of  the  kind  usual  in  alternators,  wound  in  slots  in 
the  armature  core,  and  the  leads  belonging  to  it  connect  with 
the  collecting  rings  via  the  brushes  on  the  pulley  end  of  the 
shaft. 

The  other  winding  is  a  common  continuous  current  drum- 
winding,  laid  uniformly  on  the  exterior  of  the  armature.  It  is 
provided  with  a  regular  commutator  as  shown  in  the  figure. 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  223 

The  field  is  of  laminated  iron,  and  the  field  coils  are  in  dupli- 
cate, there  being  a  coarse  wire  winding  which  in  starting  is  in 
series  with  the  commutated  armature  winding,  and  a  fine  wire 
winding  cut  out  in  starting,  but  used  alone  when  the  motor  is 
at  speed. 

Now,  when  alternating  current  is  fed  into  a  series-wound 
continuous  current  motor,  the  machine  will  start  itself  and  run, 
for  although  the  armature  current  reverses  at  all  sorts  of  times 
with  respect  to  the  winding,  the  field  also  reverses  simultane- 
ously, so  that  the  motor  tends  to  run  in  the  same  direction,  the 
relation  between  the  currents  in  armature  and  field  being  quite 
unchanged  by  the  reversal. 

The  motor  in  question  is  started  by  turning  the  alternating 
current,  reduced  to  a  moderate  voltage  by  transformation,  into 
the  series  field  and  the  commutated  winding.  The  machine 
then  starts  with  a  good  torque,  and  when  it  has  reached  syn- 
chronous speed,  indicated  by  the  pilot  lamp  on  the  top  of  the 
motor  being  thrown  into  circuit  by  a  small  centrifugal  gov- 
ernor, a  switch  is  thrown  over,  sending  the  main  current 
through  the  alternating  winding  and  closing  the  fine-wire  field 
circuit  upon  the  commutator,  at  the  same  time  cutting  out  the 
series  coils.  The  motor  then  runs  synchronously,  the  excita- 
tion being  furnished  by  the  fine  wire  winding.  This  construc- 
tion is  best  suited  for  ratner  small  machines,  as  the  double- 
winding  is  rather  cumbersome  for  large  motors. 

Except  for  inevitable  sparking  and  rather  high  inductance, 
a  series-wound  commutating  motor  could  be  successfully  used 
on  alternating  circuits,  particularly  of  low  frequency.  When 
the  series  connection  is  used  only  for  starting,  little  trouble  is 
encountered  from  these  causes. 

At  present  the  tendency  in  synchronous  motor  practice  is 
wholly  toward  the  use  of  polyphase  machines.  These  will 
start,  when  properly  designed,  as  induction  motors,  or  may  be 
started  by  separate  motors.  When  at  speed  the  field  excita- 
tion is  thrown  on,  and  the  machine  thereafter  runs  in  synchro- 
nism. Synchronous  polyphase  motors  possess  the  same  general 
properties  as  other  synchronous  motors,  and  as  most  power 
transmission  work  is  now  done  by  polyphase  currents,  they  are 
widely  used.  The  Telluride  plant  mentioned  is  now  changed 
to  the  quarter  phase  system. 


224  ELECTRIC    TRANSMISSION  OF  POWER. 

In  general  transmission  work,  synchronous  motors  find 
their  most  useful  place  in  rather  heavy  work,  which  can  be 
readily  done  at  constant  speed. 

They  have  high  power  factors  even  when  used  for  very 
varying  loads,  and  are  valuable  in  neutralizing  inductance 
in  the  line  and  the  rest  of  the  load.  Even  when  not  deliber- 
ately used  for  this  purpose,  they  raise  the  general  power  factor, 
and  thus  have  a  steadying  effect  that  is  very  useful.  When 
working  under  steady  load  and  excited  correctly,  they  almost 
eliminate  the  lagging  current  that  sometimes  becomes  so 
great  a  nuisance  in  alternating  current  working. 

The  polyphase  synchronous  motors  will  run  steadily  even 
if  one  of  the  leads  be  broken,  working  then  as  mono-phase 
machines,  and  by  stiffening  the  excitation  will  generally  carry 
their  full  normal  loads  without  falling  out  of  synchronism;  but, 
of  course,  with  increased  heating. 

In  one  case  that  came  to  the  author's  notice,  such  an  accident 
befell  a  three-phase  synchronous  motor,  which  went  quietly  on 
driving  its  load  of  1,700  looms  for  four  hours,  until  the  mill  shut 
down  at  night. 

For  small  motor  work  synchronous  machines  are  somewhat 
at  a  disadvantage,  from  the  complication  of  the  exciter  and 
inability  to  start  under  load.  In  sizes  below  100  HP  they  have 
been  very  generally  superseded  by  the  far  simpler  and  more 
convenient  induction  motor,  the  use  of  which  is  a  most  charac- 
teristic feature  of  modern  power  transmission.  In  the  use  of 
synchronous  motors,  both  monophase  and  polyphase,  there  has 
been  often  encountered  an  annoying  and  sometimes  alarming 
phenomenon  known  as  "hunting,"  or  where  several  machines 
are  involved,  as  "  pumping."  In  mild  cases  it  appears  merely  as 
a  small  periodic  variation  or  pulsation  of  the  current  taken  by 
the  motor,  often  sufficient  to  cause  embarrassing  periodic  vari- 
ations in  the  voltage  of  the  system.  The  frequency  is  ordi- 
narily one  or  two  periods  per  second,  varying  irregularly  m 
different  cases,  but  being  nearly  constant  for  the  same  machine. 
The  amplitude  may  vary  from  a  few  per  cent,  of  the  normal 
current  upwards.  Generally  the  amplitude  remains  nearly 
constant  after  the  phenomenon  is  fairly  established,  but  some- 
times it  sets  in  with  great  violence  and  the  amplitude  rapidly 
increases  until  the  motor  actually  falls  out  of  synchronism 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


22$ 


This  is  usually  the  result  of  pumping  between  two  or  more 
motors,  and  seems  to  be  especially  serious  in  rotary  convert- 
ers, not  only  throwing  them  out  of  synchronism,  but  throwing 
load  off  and  on  the  generators  with  dangerous  violence. 

Fig.  126  shows   a  facsimile   of  a  record   from  a  recording 
voltmeter  showing  the  pulsation  of  the  voltage  on  the  system 


FIG.  126. 

produced  by  the  hunting  of  a  300  HP  synchronous  motor.  It 
set  in  as  the  peak  of  the  load  came  upon  the  system  and  per- 
sisted until  the  peak  subsided,  when  it  was  gotten  under  con- 
trol, only  to  break  out  again  when  the  late  evening  load  fell  off. 
During  the  early  evening  it  was  so  severe  as  to  produce  pain- 
ful flickering  in  all  the  incandescents  on  the  circuit. 

In  this  case  the  dynamo  tender  was  inexperienced  and  had 


226  ELECTRIC    TRANSMISSION   OF  POWER. 

not  acquired  the  knack  of  so  juggling  the  field  current  as  to- 
suppress  the  hunting.  A  few  months  later  the  same  system 
was  in  regular  operation  without  the  least  trouble  from  hunt- 
ing, the  operators  by  this  time  having  been  thoroughly  broken 
in.  In  the  majority  of  cases  adroit  variation  of  the  field 
strength  abolishes  hunting,  which  almost  always  starts  with  a 
sharp  change  in  load  or  power  factor.  Just  how  to  handle  the 
excitation  to  obtain  the  best  results  is  a  matter  of  experiment 
in  each  particular  case,  but  except  in  cases  of  unusually  seri- 
ous character  the  knack  is  soon  acquired.  A  rather  strong 
field  often  steadies  things,  although  if  strong  enough  to  pro- 
duce leading  current  the  trouble  is  sometimes  aggravated. 
But  in  this  case,  as  in  operating  alternators  in  parallel,  the 
best  running  conditions  have  to  be  learned  by  experience. 

Much  yet  remains  to  be  learned  about  the  exact  nature  of 
hunting,  but  its  general  character  is  about  as  follows:  A 
sudden  change  in  current  or  phase  causes  the  armature  to  seek 
a  new  position  of  equilibrium.  In  so  doing  the  sudden  change 
iin  the  armature  reaction  momentarily  changes  the  field 
strength,  which  aggravates  the  instability  already  existing  and 
causes  the  armature  under  the  influence  of  its  own  inertia  to 
overreach  and  run  beyond  its  normal  position  of  equilibrium. 
Then  the  field  recovers  and  the  armature  swings  back,  once 
more  shifting  the  field  and  again  overrunning,  and  so  on  ad 
nauseam.  The  pulsation  of  the  exciting  current  in  cases  of 
hunting  is  generally  very  conspicuous,  and  the  periodicity  of 
the  hunting  seems  to  correspond  in  general  with  the  time  con- 
stant of  the  field  magnetization. 

A  fly-wheel  on  the  motor  or  direct  connection  to  a  heavy 
machine  generally  increases  the  trouble,  while  belted  and  flex- 
ibly connected  motors  suffer  less.  Heavy  drop  in  the  supply 
lines,  which  makes  the  voltage  at  the  motor  sensitive  to  varia- 
tions of  current,  and  low  reactance  in  the  armature,  which 
favors  large  fluctuations  of  current,  are  conditions  specially 
favorable  to  violent  hunting.  Rotary  converters  in  which  the 
armature  current  and  its  reactions  are  very  heavy  compared 
with  that  component  of  the  current  which  is  directly  concerned 
with  the  rotation  of  the  machine  as  a  synchronous  motor,  are 
subject  to  peculiarly  vicious  hunting,  which  has  often  risen  to- 
the  point  where  it  threw  the  rotary  out  of  synchronism. 


SYNCHRONOUS  AND  INDUCTION  MOTOXS.  227 

They  are  far  less  stable  in  this  particular  than  ordinary  syn- 
chronous motors,  and  cannot  readily  be  controlled  by  varying" 
the  excitation  on  account  of  the  consequent  variation  of  volt- 
age on  the  continuous  current  side. 

Motors  and  rotaries  having  their  pole  pieces  not  laminated, 
but  solid,  often  show  less  tendency  to  hunt  than  machines  with 
laminated  poles.  If  the  poles  are  solid  any  violent  swaying  of 
the  armature  current  with  reference  to  them  is  checked  and 
damped  by  the  resulting  eddy  currents,  so  that  the  hunting  is 


FIG.  127. 

pretty  effectively  choked.  For  the  same  reason  alternators- 
in  parallel  are  less  likely  to  pump  if  they  have  solid  poles,  and 
most  foreign  machines  are  built  in  such  wise.  Here,  Where 
laminated  poles  are  just  now  the  rule,  recourse  is  had  to 
"  bridges  "  or  "shields."  These  are  essentially  heavy  flanges 
of  copper  or  bronze  attached  to  the  edges  of  the  poles  so  that 
fluctuations  of  armature  reaction  and  of  field  are  damped  by 
heavy  eddy  currents  whenever  they  arise.  An  example  of  such 
practice  is  shown  in  Fig.  127,  which  shows  a  portion  of  the  re- 
volving field  of  a  large  polyphase  generator  fitted  with  massive 
castings,  bridging  the  spaces  from  pole  piece  to  pole  piece  and 
serving  at  once  to  hold  the  field  coils  rigidly  wedged  into  place 
and  to  check  pumping  when  the  generators  are  running  in  par- 
allel. A  similar  device  is  used  in  connection  with  many  rotary 


228  ELECTRIC    TRANSMISSION  OF  POWER. 

converters  with  a  very  fair  degree  of  success.  Occasionally 
pumping  may  be  traced  to  some  definite  cause  like  a  defective 
engine  governor  having  a  periodic  vibration,  but  more  often 
the  phenomenon  is  purely  electromagnetic.  The  use  of 
shields  or  solid  pole  pieces  constitutes  the  best  general  remedy, 
for,  while  adjustment  of  the  field  is  often  effective,  it  is  often 
desirable  to  adjust  the  field  for  other  purposes,  and  the  neces- 
sity of  varying  it  to  suppress  hunting  is  sometimes  very  em- 
barrassing, if  not  impossible. 

INDUCTION    MOTORS. 

An  induction  motor  is  a  motor  into  which  working  current 
is  introduced  by  electromagnetic  induction  instead  of  by 
brushes.  It  has  therefore  two  distinct,  although  co-ordinated, 
functions — transformer  and  motor.  To  understand  its  action 
we  must  take  care  not  to  confuse  these  functions,  and  this  is 
best  done  by  recurring  to  the  fundamental  principles  that  are 
at  the  root  of  all  motors  of  whatever  kind. 

An  electric  motor  consists  of  these  essential  parts,  viz.:  A 
magnetic  field,  a  movable  system  of  wires  carrying  electric 
currents  and  means  for  organizing  these  two  elements  so  as 
to  produce  continuous  torque. 

These  parts  are  beautifully  shown  in  their  elementary 
simplicity  in  Barlow's  wheel,  Fig.  128,  invented  nearly  three- 
quarters  of  a  century  ago. 

In  this  machine  N  S  is  the  permanent  field-magnet,  the 
arms  of  the  star-shaped  wheel  are  the  current-carrying  con- 
ductors, and  a  little  trough  placed  between  the  magnet  poles, 
and  partly  filled  with  mercury,  serves  with  the  wheel  as  a  com- 
mutator. Its  function  is  to  shift  the  current  from  one  con- 
ductor to  the  next  following  one,  when  the  first  passes  out  of 
an  advantageous  position.  In  other  words  it  keeps  the  cur- 
rent flowing  so  as  to  produce  a  constant  torque,  irrespective  of 
the  movement  of  the  conductors.  Such  is  precisely  the  func- 
tion of  the  modern  commutator,  and  it  is  interesting  to  note 
that  the  device  of  making  the  armature  conductors  themselves 
serve  as  the  commutator  is  successfully  used  in  some  of  the 
best  modern  machines. 

These  same   fundamental   parts  are  found  alike  in  motors 


SYNCHRONOUS  AND  INDUCTION  MOTORS.  229 

designed  for  continuous  or  for  alternating  currents.  We  have 
already  seen  that  a  series-wound  motor  can  serve  for  use  with 
both  kin^s  of  current,  since  the  commutator  distributes  the 
current  alike  for  both,  and  since  the  direction  of  the  torque  is 
determined  by  the  relative  direction  of  the  main  field  and  that 
due  to  the  moving  conductors,  alternations  which  affect  both 
symmetrically  leave  the  torque  unchanged. 

We  have  seen  also  that  if  the  distribution  of  currents  given 
by  the  commutator  can  be  simulated  by  supplying  the  arma- 
ture with  alternating  impulses  timed  as  the  commutator  would 


time  them,  we  can  dispense  with  the  commutator,  and  sub- 
stitute two  slip  rings.  In  this  case,  however,  the  motor  will 
only  run  when  in  synchronism,  since  then  only  will  the  alternat- 
ing impulses  from  the  generator  be  properly  distributed  in  the 
armature,  as  has  already  been  explained.  Besides,  the  current 
has  to  be  introduced  into  the  armature  through  brushes  bear- 
ing on  a  pair  of  slip  rings,  and  an  exciter  is  required  to  supply 
the  field.  If  one  could  use  an  alternating  field,  and  induce  the 
currents  in  the  armature  as  one  would  in  the  secondary  of  a 
common  transformer,  the  machine  would  be  of  almost  ideal 
simplicity. 

This  is  what  is  accomplished  in  the  induction  motor.  The 
field  is  supplied  with  alternating  current,  and  the  working  cur- 
rent is  induced  directly  in  the  armature  conductors. 

To  this  end  the  brushes  used  in  the  previous  examples  may 
be  replaced  by  a  pair  of  inducing  poles,  carrying  the  primary 
windings,  to  which  the  armature  windings  play  the  r6Ie  of 
secondary.  These  irmature  windings  are  therefore  closed  on 
themselves,  instead  of  being  brought  out  to  slip  rings 

For  this  short-circuited  winding  various  forms  are  employed, 
the  simplest  being  shown  in  Fig.  129.  It  consists  of  a  set  of 
copper  bars  thrust  through  holes  near  the  periphery  of  the 


230 


ELECTRIC   TRANSMISSION  OF  POWER. 


laminated  armature  core,  and  all  connected  together  at  each 
end  by  heavy  copper  rings. 

The  simplest  arrangement  of  field  and  inducing  poles  is 
shown  in  Fig.  130.  Here  each  pair  of  opposite  poles  is  provided 
with  a  separate  winding,  so  that  the  circuit  A  A  supplies  alter- 
nating current  to  one  pair  and  B  B  to  the  other  pair.  The 
armature  we  will  assume  to  be  like  Fig.  129.  Now  apply  an 


FIG.  129. 

alternating  current  to  A  A.  The  windings  of  the  armature 
which  enclose  the  varying  electromagnetic  stress  will  have  set 
up  in  them  a  powerful  alternating  current  almost  180°  behind 
the  primary  current,  /.  <?,  in  general  opposed  to  it  in  direction,  as 
considerations  of  energy  require.  The  armature  will  not  turn, 
however,  for  two  very  good  reasons:  first,  the  current  in  it  is 
far  out  of  phase  with  the  magnetization  of  the  poles;  and 
second,  this  current  is  quite  symmetrical  with  respect  to 
the  poles,  so  that  the  only  effect  could  be  a  straight  push  or 
pull  without  the  slightest  tendency  to  attract  or  repel  one 
side  of  the  armature  more  than  the  other. 

To  produce  rotation  as  a  motor,  there  must  be  not  only  a 
current  in  the  armature  conductors,  but  there  must  be  field 
poles  magnetized  and  disposed  so  as  to  produce  a  torque  upon 
these  conductors. 

Suppose,  now,  an  alternating  current  to  be  sent  around  the 
circuit  B  B.  If  it  is  applied  simultaneously  with  the  current 
in  A  A,  we  shall  be  no  better  off  than  before,  for  since  the  two 
pairs  of  poles  act  together  and  just  alike,  there  is  no  magnetiza- 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


231 


tion  in  phase  with  the  armature  current,  and  nothing  to  cause 
the  armature  to  turn  either  way. 

To  obtain  rotation  we  must  arrange  the  two  sets  of  poles  so 
that  one  pair  may  furnish  a  magnetic  field  with  which  the  cur- 
rent induced  by  the  other  pair  is  able  to  react.  The  simplest 
way  of  doing  this  is  to  supply  B  B  with  current  90°  in  phase 
behind  the  current  in  A  A.  Then  when  the  current  induced 
by  A  A  rises,  it  finds  the  poles  B  B  energized  and  ready  to 


FIG.  130 

attract  it,  for  the  magnetization  in  B  B  and  the  current  are 
less  than  90°  apart  in  phase.  The  less  the  lag  of  the  arma- 
ture current  behind  its  E.  M.  F.,  the  more  nearly  will  the 
magnetization  of  these  field  poles  be  in  phase  with  the 
armature  current,  and  the  more  powerful  will  be  the  torque 
produced. 

The  B  B  set  of  poles  necessarily  induce  secondary  currents  in 
the  armature  in  their  turn,  toward  which  the  A  A  poles  serve 
as  field  during  the  next  alternation.  The  directions  of  both 
armature  current  and  field  magnetization  are  now  reversed, 
so  that,  as  in  the  commutating  motor,  the  torque  is  unchanged. 
The  next  alternation  begins  the  cycle  over  again,  and  so  the 
motor  runs  up  to  speed.  Its  direction  of  rotation  depends 
evidently  upon  the  relative  directions  of  magnetization  in  the 
two  sets  of  poles,  for  these  determine  the  direction  of  the 
armature  current  and  the  nature  of  the  field  poles  that  act 


232  ELECTRIC    TRANSMISSION   OF  POWER. 

upon  it.  Reversing  the  current  in  A  A  or  B  B  will  therefore 
reverse  the  motor,  while  reversing  both  will  not. 

The  speed  of  the  armature  is  determined  in  a  rather  inter- 
esting manner.  When  the  armature  is  in  rotation  the  electro- 
magnetic stresses  which  act  upon  a  given  set  of  armature 
conductors  are  subject  to  variation  from  two  causes.  First  is 
the  variation  in  magnetization,  due  to  changes  in  the  primary 
current;  second,  the  variation  due  to  the  armature  coils  mov- 
ing as  the  armature  turns,  so  as  to  include  more  or  less  of  the 
magnetic  stress.  The  E.  M.  F.  in  the  armature  conductors  is 
due  to  the  summed  effect  of  these  two  variations.  And  since 
the  two  are  in  opposition,  if  the  armature  were  moving  fast 
enough  to  make  a  half  revolution  for  each  alternation  of  the 
field,  the  E.  M.  F.  produced  would  be  zero,  since  the  rates  of 
change  in  the  field  and  in  the  area  of  stress  included  by  the 
armature  coils  would  be  equal. 

This  means  that  the  armature  must  always  run  at  less  than 
synchronous  speed — enough  less  to  produce  a  net  armature 
E.  M.  F.  high  enough  to  give  sufficient  armature  current  for 
the  torque  needed. 

Under  varying  loads,  therefore,  an  induction  motor  behaves 
much  like  a  shunt-wound  continuous  current  motor.  In  both, 
the  armature  current  is  due  to  the  net  effect  of  an  applied  and 
a  counter  E.  M.  F.,  the  former  being  delivered  from  the  line 
through  brushes  in  the  one  case  and  by  induction  in  the 
other.  In  neither  case  can  the  speed  rise  high  enough  to 
equalize  these  two  E.  M.  Fs.  There  is,  however,  a  very  curi- 
ous and  interesting  form  of  induction  motor  which  runs  at 
true  synchronous  speed  until  the  load  upon  it  reaches  a  certain 
point,  when  it  falls  out  of  step  like  any  other  synchronous 
motor,  or  under  certain  circumstances  falls  out  of  synchronism 
and  then  operates  like  an  ordinary  asynchronous  motor. 

Its  operation  in  synchronism  seems  a  paradox  at  the  first 
glance,  but  the  principle  involved  is  really  simple,  although 
the  exact  theory  of  the  motor  is  a  bit  complicated.  As 
has  already  been  noted,  if  the  rates  of  change  of  magnetic 
induction  due  to  the  pulsation  of  the  field  and  to  the  cutting 
of  the  field  by  the  armature  coils  are  equal  and  opposite,  there 
will  be  no  E.  M.  F.  in  these  coils,  and  obviously  no  energy 
can  be  transferred  from  field  to  armature.  If,  however,  the 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  233 

E.  M.  F.  wave  due  to  the  change  of  magnetization  in  the  field 
and  that  due  to  the  motion  of  the  armature  coils  through  th.e 
field  are  very  different  in  shape,  there  can  still  be  a  periodical 
resultant  E.  M.  F.,  generally  of  a  very  complicated  description, 
accompanied  by  a  transfer  of  energy  even  at  full  synchronous 
speed.  A  very  irregular  wave  shape  in  the  E.  M.  F.  of  supply, 
or  a  distortion  of  it  due  to  extraordinary  armature  reactions, 
may  produce  this  condition.  Fig.  131  shows  the  primary  E. 


FIG.  131. 

M.  F.  wave  form  as  taken  by  the  oscillograph  across  the 
terminals  of  such  a  synchronous  induction  motor,  and  the  cor- 
responding current  wave,  which  emphasizes  the  significance  of 
the  facts  just  given.  The  condition  is  best  reached  in  small 
motors  having  sharply  salient  field  poles.  The  writer  has 
never  seen  one  which  would  start  from  rest  unaided,  the  great 
field  distortion  necessary  being  in  the  way,  but  once  spun  up 
to  or  near  synchronism  they  work  admirably  on  a  small  scale. 
The  conditions  of  energy  supply  are  obviously  such  as  to  be 
highly  unfavorable  in  motors  of  any  size,  but  for  laboratory  or 
other  purposes  where  synchronous  speed  is  wanted  they  are 
very  convenient  for  an  output  of  -|  HP  or  so,  and  form  a 
very  striking  modification  of  the  ordinary  induction  motor. 
They  have,  up  to  the  present,  been  made  mostly  by  the  Holtzer- 
Cabot  Electric  Co. 

If  the  load  on  an  induction  motor  increases,  demanding  an 
increased  torque,  the  armature  slows  down  a  trifle,  until  the  new 
armature  E.  M.  F.  and  resulting  current  are  just  sufficient  to 
meet  the  new  conditions.  In  the  continuous  current  motor 
this  speed  is  determined  by  the  resistance  of  the  armature,  to 
which  the  current  corresponding  to  a  givsn  decrease  of  speed 


234 


ELECTRIC    TRANSMISSION   OF  POWER. 


is  necessarily  proportional.  In  the  induction  motor  the  arma- 
ture resistance  plays  a  precisely  similar  role.  Fig.  132  shows 
the  actual  speed  variation  of  a  100-  HP  induction  motor  in  terms 
of  its  output.  The  maximum  fall  in  speed  under  full  load  is  a 
trifle  less  than  3  percent.,  and  even  this  result  is  sometimes 
surpassed  in  induction  motors  for  especial  purposes,  even  a  i 
percent,  variation  having  been  reached.  A  motor  with  higher 
armature  resistance  would  fall  more  in  speed,  like  a  shunt 
motor  with  a  rather  high  armature  resistance.  We  thus  see 
that  the  induction  motor,  as  it  should,  behaves  much  like  any 
other  motor;  the  torque  is  produced  in  the  same  way,  and 
obeys  similar  laws;  the  motor  is  similarly  self-starting,  and 
works  on  the  same  general  principles  throughout.  Obviously 
the  magnitude  of  the  armature  current  in  an  induction  motor  is 


30  40  50  60 

HP.  OUTPUT 

FIG.  132. 

determined,  not  by  the  armature  resistance  alone,  but  by  its 
impedance.  As,  however,  the  presence  of  reactance  shifts  the 
phase  of  the  current,  and  that  component  of  current  which  is 
effective  in  producing  torque  depends  upon  the  resistance, 
the  relation  just  explained  holds  good.  That  current  is  deliv- 
ered to  the  armature  by  induction  is  a  striking  feature,  but 
not  one  that  implies  any  radical  difference  in  principle. 

It  is  not  even  necessary  to  use  a  polyphase  circuit  for  work- 
ing induction  motors,  for,  under  certain  conditions,  the  same 
set  of  poles  can  perform  the  double  duty  of  delivering  current 
and  interacting  with  it  to  produce  torque. 

The  principles  of  the  induction  motor,  as  here  given,  thus 
become  part  of  the  general  theory  of  the  electric  motor  which 
applies  alike  to  machines  for  continuous  and  alternating  cur- 
rent, quite  independent  of  particular  methods  of  construction 
or  operation. 

The  great  pioneers  in  induction  motor  work,  Tesla,  Ferraris, 


SYNCHRONOUS   AND  INDUCTION  MOTORS. 


235 


and  some  others,  preferred  to  view  the  matter  from  the  special 
rather  than  the  general  standpoint,  and  hold  to  the  theory  of 
the  rotary  pole  action  of  induction  motors— very  beautiful, 
mathematically,  but  unfortunately  hiding  the  kinship  of  induc- 
tion to  other  motors,  and  distracting  attention  from  the  trans- 
former action,  which  is  so  prominent. 

From  this  point  of  view  the  two  pairs  of  poles  in   Fig.  130 


FIG.  133. 


FIG.  134. 


FIG.  135. 


co-act  to  produce  an  oblique  resultant  magnetization,  which 
shifts  around  the  field,  producing  a  moving  system  of  poles, 
following  the  sequence  of  the  current  phases,  and  dragging 
around  the  armature  after  them,  by  virtue  of  the  currents  in- 
duced in  it.  Figs.  133,  134,  135  show  the  rudimentary  prin- 
ciples of  th6  rotary  pole.  In  Fig.  133  an  annular  field  magnet 
is  wound  with  two  circuits  A  A  and  B  B,  supplied  with  alter- 
nating currents  90°  apart  in  phase.  The  polarity  of  the 
armature  is  represented  diagramatically  by  the  rotating 
magnet  N  S. 

Now,  when  the  current  in  A  A  is  maximum  (and  that  in  B  B 
is  consequently  zero),  the  field  has  poles  at  P  and  Pt'  which 


236  ELECTRIC    TRANSMISSION  OF  POWER. 

exert  a  torque  on  the  armature  poles.  As  the  current  falls  in 
A  A  and  rises  in  B  B,  the  resultant  poles  move  forward  to  Pl 
and  j/Y  (Fig.  134),  followed  by  the  armature.  When  the  cur- 
rent B  B  is  a  maximum,  and  A  A  has  become  zero,  the  poles  are 
at  P9  and  P9'  and  so  on.  In  order  that  the  revolving  poles 
may  induce  current  in  the  armature,  the  latter  must  slip 
behind  so  as  to  produce  relative  motion  and  change  in  electro- 
magnetic stress. 

This  point  of  view  is  very  interesting  and  instructive.  It 
deals, however,  not  directly  with  the  two  field  magnetizations, — 
the  functions  of  which  have  just  been  discussed, — but  with  a 
resultant  rotary  magnetic  field,  which  may  or  may  not  have  a 
concrete  existence,  according  to  circumstances.  It  by  no 
means  follows  that  because  two  equal  energizing  currents  are 
90°  apart  in  phase,  they  must  or  do  form  a  resultant  rotary 
magnetic  field,  or  that,  if  they  are  so  organized  as  to  give  a 
physical  resultant,  their  individual  functions  are  superseded 
and  must  be  neglected. 

The  two  views  of  the  induction  motor  here  set  forth  are  not 
in  any  way  conflicting;  they  merely  represent  two  methods  of 
treatment  of  the  same  phenomena.  As  it  happens,  the  rotary 
field  point  of  view  is  from  a  mathematical  standpoint  the 
easier,  for  it  treats  the  resultant  instead  of  its  components, 
and  hence  has  been  the  oftener  used,  but  in  discussing  certain 
classes  of  induction  motors  it  is  by  no  means  convenient,  and 
is  less  general  than  the  analytical  method,  which  deals  with  the 
separate  components.  In  most  commercial  induction  motors, 
there  is  undoubtedly  a  resultant  rotary  field,  but  however  con- 
venient  it  may  be  to  consider  the  motors  in  that  light,  it  is  not 
well  to  lose  sight  of  the  general  actions  of  which  the  rotary- 
field  is  a  special  case. 

As  a  matter  of  fact,  the  several  currents  in  a  polyphase  in- 
duction motor  may  be  so  distributed  that  they  cannot  produce 
a  resultant  rotary  magnetization,  and  in  certain  heterophase 
and  monophase  motors  the  "  rotary  field,"  in  so  far  as  one  is 
formed  by  the  field,  may  revolve  in  one  direction  while  the 
armature  starts  and  runs  strongly  in  the  other  direction. 
Hence  the  view  here  taken  of  the  induction  motor  has  been 
generalized  for  the  purpose  of  bringing  out  its  relation  to  the 
general  theory  of  motors,  and  to  take  account  of  induction 


SYNCHRONOUS  AND   INDUCTION  MOTORS, 


237 


motors  in  explaining  which  the  rotary  pole  theory  would  have 
to  be,  as  it  were,  dragged  in  by  the  ears. 

Salient  poles,  like  those  of  Fig.  130,  are  seldom  used,  and 
the  induction  motor,  as  generally  constructed,  consists  of  two 
short  concentric  cylinders  of  laminated  iron,  slotted  on  their 
opposed  faces  to  receive  the  windings.  Sometimes  these  slots 
are  open,  and  again  they  are  simply  holes  close  to  the  surface 
of  the  iron. 


FIG.  136. 

The  relation  of  the  parts  is  well  shown  in  Fig.  136,  a  6  HP 
two-phase  motor  by  C.  E.  L.  Brown. 

In  this  case  the  exterior  ring  is  the  primary,  and  the  revolv- 
ing ring  the  secondary,  element  of  the  motor.  The  primary 
winding  is  of  coils  of  fine  wire  threaded  through  the  core 
holes,  while  the  secondary  member  is  wound,  if  one  may  use 
the  term,  with  solid  copper  rods  united  at  the  ends  by  a 
broad  copper  ring.  The  clearance  between  primary  and 
secondary  is  very  small  in  all  induction  motors,  almost  always 


238 


ELECTRIC    TRANSMISSION  OF  POWER. 


less  than  \  inch,   sometimes  less  than  ^  inch.     The  smaller 
the  clearance  the  better  the  machine  as  a  transformer. 

The  primary  of  an  induction  motor  is  wound  much  as  the 
armature  of  a  polyphase  generator  is  wound,  as  described 
already.  Fig.  137  shows  in  diagram  a  two-phase  winding  for  a 
24  slot  primary,  and  Fig.  138  a  three-phase  winding  for  the 


FIG.  137. 

same  primary.  In  the  former  there  are  two  sets  of  coils  A 
and  B,  each  forming  a  separate  phase  winding;  in  the  latter  the 
three  sets  A,  B,  C,  may  be  united  to  form  either  a  "star"  or 
"  mesh  "  three-phase  winding.  In  practice  the  primary  winding 
is  nearly  always  polyodontal,  for  the  same  general  reasons  that 
hold  for  generator  armatures,  but  especially  to  keep  down 
inductance.  For  the  same  reason  the  secondary  winding  is 
polyodontal.  As  an  example  of  the  best  usage  in  this  respect, 
Fig.  140  shows  the  number  and  relation  of  primary  and 
secondary  slots  in  the  motor  shown  in  Fig.  136.  There  are  no 
less  than  40  primary  slots  for  a  four-pole  winding,  /.  e.,  5  slots 
per  phase  per  pole,  while  the  secondary  has  37  slots,  this  odd 
number  being  chosen  to  reduce  the  variation  in  the  magnetic 
relations  of  primary  and  secondary  due  to  different  positions 
of  the  armature. 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


239 


Induction  motors  with  fixed  primary  have  the  great  advan- 
tage of  having  no  moving  contacts,  and  no  high  voltage  wind- 
ings exposed  to  the  strains  due  to  revolution.  On  the  other 
hand  a  revolving  primary  makes  it  very  easy  to  vary  the  resist- 
ance in  the  secondary  circuit,  which  is  often  desirable.  Both 
forms  are  used,  the  latter  only  rarely.  Inasmuch  as  a  large 
proportion  of" the  hysteretic  loss  occurs  in  the  primary,  since 
in  the  secondary  the  variation  of  the  magnetization  is  small, 
a  revolving  primary,  being  of  less  dimensions  than  its  second- 
ary, gives  a  slight  advantage  in  efficiency.  There  is,  however, 
small  reason  to  suppose  that  on  the  whole  it  is  easier  to  build 
one  form  than  the  other  for  a  given  efficiency  with  the  same 
care  in  designing. 


FIG.  138. 

Plate  V  shows  a  pair  of  induction  motors  of  the  American 
types.  Fig.  i  is  a  15  HP  two-phase  Tesla  motor,  manufactured 
by  the  Westinghouse  Company.  The  primary  is  the  revolving 
member,  and  receives  current  via  the  slip  rings  just  outside  the 
bearing.  If  resistance  is  to  be  inserted  in  the  secondary,  it 
can  be  done  with  a  stationary  rheostat. 

Fig.  2  is  a  125  HP  three-phase  motor,  made  by  the  General 
Electric  Company.  In  this  machine  the  secondary  rotates. 


240 


ELECTRIC    TRANSMISSION  OF  POWER. 


The  short  lever  alongside  the  bearing  moves  a  loose  ring  aiong 
the  shaft,  and  thus  serves  to  cut  in  or  out  a  resistance  in  the 
secondary,  which  resistance  is  carried  on  the  armature  spider, 
and  is  generally  in  two  or  three  sections,  thrown  in  or  out 
consecutively.  With  the  lever  placed  as  shown,  all  the 
secondary  resistance  is  in  circuit. 

A  glance  at  the  plate  shows  that  the  secondary  windings  are 
by  no  means  as  simple  as  that  of  Fig.  129.  They  are  true 
bar  windings,  not  merely  bars  with  a  common  connection.  At 
the  present  time  both  these  forms  of  motor  have  been  super- 


CONNECTIONS  FOR  AUTO-STARTER 
CIRCUIT 


STARTING  POSITION 


AUTO-STARTER  IN  STARTING  POSITION 


THHEE  PHASE 

MOTOR. 
AUTO-STARTER  IN,  RUNNING  POSITION) 


FIG.  139. 

seded,  but  they  are  of  interest  in  showing  the  progress  of  the 
art.  Motors  with  revolving  primary  are  no  longer  regularly 
manufactured,  the  superior  simplicity  of  the  other  construction 
being  generally  recognized.  Plate  VI  shows  the  motors 
now  in  common  use  in  this  country.  Fig.  i  is  a  standard 
Westinghouse  "Type  C  "  motor  of  20  HF.  It  is  exceedingly 
simple  in  construction,  and  efficient  in  operation.  It  has  a 
"squirrel-cage"  armature  similar  to  that  of  Fig.  129,  but  the 
bars  are  in  open  slots  and  are  of  rectangular  section,  a 
construction  which  gives  a  lower  armature  reactance  than  if 
the  iron  were  closed  over  the  armature  bars.  These  motors 
start  with  a  powerful  torque,  approximately  three  times  the 
torque  at  rated  full  load,  when  the  full  line  voltage  is  thrown 


PLATE    VI. 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  241 

upon  the  primary,  but  of  course  take,  under  these  conditions, 
a  very  heavy  current,  so  that  in  practice  it  is  usual  to  start 
them  at  reduced  voltage,  which  gives  all  the  torque  necessary 
without  calling  for  excessive  current.  This  is  accomplished 
by  means  of  a  so-called  auto-converter,  of  which  the  essential 
•connections  are  shown  in  Fig.  139.  With  the  switch  in  the 
starting  position  the  applied  voltage  is  only  a  quarter  or  a  half 
the  normal  voltage,  the  actual  amount  being  adjusted  by 
means  of  the  variable  connections  shown,  and  when  the  motor 
has  come  up  to  its  full  speed  under  the  starting  conditions  the 
switch  is  suddenly  thrown  over,  putting  the  full  working  volt- 
age in  circuit.  It  is  necessary  to  let  the  motor  reach  its  full 
speed  with  the  lower  voltage  before  making  this  change,  else 
there  will  be  a  needlessly  severe  current  due  to  the  sudden 
acceleration  under  full  voltage,  and  the  change  should  be 
made  quickly,  lest  the  armature  speed  should  fall  off  during 
the  change  and  produce  the  same  unpleasant  result.  When 
intelligently  handled  the  starting  current  can  be  kept  within 
very  reasonable  limits,  but  the  auto-converter  should  be  ad- 
justed when  set  up  to  give  at  starting  merely  the  voltage 
needed  to  start  under  the  required  torque,  an  excess  of  voltage 
meaning  excess  of  current.  Fig.  2  of  Plate  VI  is  a  General 
Electric  "Type  L"  motor  of  10  HP,  having  characteristics 
similar  to  the  motor  Fig.  2  of  Plate  V.  The  special 
peculiarity  of  the  later  form  is  the  arrangement  of  the  starting 
resistance  in  the  secondary.  This  is  operated  by  a  rod  at- 
tached to  the  knob  which  appears  at  the  end  of  the  armature 
shaft.  The  shaft  is  hollow  and  the  rod  is  attached  through 
a  slot  to  the  short-circuiting  brushes,  which  bear  upon  the 
resistances  stowed  away  within  the  armature  spider.  These 
resistances  are  in  many  sections  and  can  be  short-circuited 
very  gradually,  holding  the  primary  current  practically  con- 
stant from  start  to  full  speed  even  when  starting  under  a 
heavy  torque.  The  start  is  made  with  the  rod  pulled  out  to  its 
fullest  extent,  and  it  is  gradually  pushed  or  tapped  home  until 
full  speed  is  reached.  Such  motors  are  peculiarly  well  adapted 
for  use  on  lighting  circuits,  and  in  large  sizes  requiring  heavy 
starting  torque.  The  start  can  be  made  with  very  moderate 
currents,  and  the  torque  per  ampere  is  considerably  greater 
than  in  any  motor  starting  on  reduced  primary  voltage,  which 


242  ELECTRIC    TRANSMISSION  OF  POWER. 

is  the  compensation  for  the  rather  elaborate  starting  device. 
Neither  of  the  companies  mentioned  holds  rigidly  to  the  con- 
structions here  shown,  but  the  cuts  show  their  best  standard 
practice.  There  is  very  little  difference  in  the  essential  prop- 
erties of  the  two  forms,  and  both  are  very  widely  used. 

These  recent  motors  are  nearly  all  made  with  extremely 
small  clearance  between  armature  and, field,  from  y1^  to  -^  inch 
or  less,  even  in  large  motors.  This  practice  renders  it  easy  to 
design  for  a  good  power  factor,  but  may,  and  sometimes  does, 
cause  trouble  mechanically,  as  might  be  anticipated.  It  is  not 
difficult  to  make  thoroughly  good  motors  without  resorting  to 
such  extreme  measures,  unless  the  designer  is  hampered  by 
troublesome  specifications  in  other  particulars.  Demand  for 
slow  speed  motors  at  a  periodicity  of  6o~,  and  insistence  on  a 
uniformity  of  speed  at  various  loads  that  would  not  for  a  mo- 
ment be  demanded  in  direct  current  motors,  are  responsible 
for  serious  and  needless  impediments  in  induction  motor 
design. 

The  "  Type  L  "  motors  just  described  have  on  the  armature 
a  regular  three-phase  winding  of  rectangular  bars  united  by 
end  connectors.  A  simple  four-pole  form  of  such  a  winding  is 
shown  in  Fig.  141.  It  obviously  is  more  troublesome  to  con- 
struct than  a  "  squirrel-cage  "  winding,  but  it  possesses  certain 
advantages.  Conspicuously,  it  renders  it  possible  to  insert 
resistance  in  the  secondary  circuit  at  starting,  which  in  the 
"squirrel-cage"  would  be  a  very  difficult  matter,  although  it 
has  been  tried. 

If  the  field  is  very  uniform,  with  a  thoroughly  distributed 
winding,  there  is  very  little  difference  in  the  actual  perform- 
ance of  the  two  kinds  of  armatures  (drum-wound  and  "squir- 
rel-cage ")  when  at  speed.  In  case  of  a  motor  with  salient 
poles  or  with  few  winding  slots  in  the  field,  the  drum  armature 
has  a  very  considerable  advantage,  owing  to  the  fact  that  the 
currents  in  it  are  directed  into  definite  paths  which  they  must 
follow  at  all  times,  while  in  the  "  squirrel-cage  "  form  the  cur- 
rents are  only  uniformly  organized  when  there  is  a  uniform 
field.  In  the  early  motors,  therefore,  the  drum-wound  arma- 
ture nad  a  great  advantage,  but  as  the  art  of  designing  has 
advanced  the  two  types  have  become  closely  approximated  in 
their  properties. 


SYNCHRONOUS  AND  INDUCTION  MOTORS. 


243 


In  this  country  the  windings  of  induction  motors  are  gener- 
ally placed  in  open  or  nearly  open  slots,  as  in  the  case  of  the 
motors  shown  in  Plate  VI.  Abroad  the  arrangement  of  wind- 
ings in  holes  as  shown  in  Fig.  140  is  very  common.  Each 
procedure  has  its  advantages.  The  American  practice  renders 
it  very  easy  to  place  the  windings,  and  to  put  a  very  large 
amount  of  copper  upon  the  armature,  for  open  slots  can  be 
made  radially  deep  and  filed  true,  while  holes  unless  rather 


FIG.  140. 

large  can  only  be  trued  by  reaming,  which  implies  a  round 
hole,  unfavorable  if  a  great  amount  of  copper  is  to  be  crowded 
upon  the  armature.  Hence  with  open  slots  it  is  easier  to  sub- 
divide the  winding  into  many  slots,  thus  reducing  the  armature 
reactance.  On  the  other  hand,  open  slots  are  extremely  unfa- 
vorable as  regards  power  factor,  since  the  iron  surfaces  opposed 
in  armature  and  field  are  very  greatly  reduced,  and  hence  the 
tendency  to  use  extremely  small  clearances  in  order  to  make 
the  best  of  a  bad  matter.  The  European  practice  is  on  the 
whole  better  as  regards  power  factor,  but  does  not  facilitate 
the  construction  of  motors  of  very  low  armature  resistance, 


244 


ELECTRIC   TRANSMISSION  OF  POWER. 


and  is  considerably  more  difficult  of  proper  execution.  The 
matter  really  hinges  on  the  relative  cost  of  labor  here  and 
abroad.  With  cheap  labor  the  manufacturer  can  afford  to  go 
into  little  refinements  if  it  is  otherwise  worth  while,  but  at 
American  labor-rates  handwork  has  to  be  minimized.  On  the 
whole,  the  American  motors  are  fully  up  to  foreign  standards 
in  general  design,  although  the  tendency  here  has  been  to 
made  a  fetish  of  uniformity  of  speed,  even  at  the  expense  of 
more  important  characteristics. 


FIG,  141. 

In  motors  such  as  those  just  described,  with  distributed 
windings  and  no  sharply  defined  polar  areas,  the  consecutive 
exchange  of  motor  and  transformer  functions  among  the  wind- 
ings is  almost  lost  sight  of  in  the  presence  of  the  very  apparent 
phenomenon  of  resultant  revolving  poles,  but  the  appearance 
of  the  latter  is  a  necessary  result  of  the  persistence  of  the 
former.  These  induction  motors  are  generally  operated  from 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


245 


the  secondary  circuits  of  transformers,  although  the  large 
sizes  (50  HP  and  upward)  are  sometimes  wound  for  use  of 
the  full  primary  voltage  up  to  2,000  volts  or  more. 

Another  form  of  induction  motor  which  possesses  some 
interesting  features  is  the  Stanley  machine,  shown  in  Figs.  142, 
143.  The  field  shown  in  Fig.  142  is  composed  of  two  separate 
rings  of  laminated  iron,  each  having  eight  polar  projections. 
These  field  rings  are  assembled  side  by  side  with  the  poles 


FIG.  142. 


"staggered,"  as  shown  in  the  cut.  Each  field  is  energized 
separately,  one  from  each  branch  of  a  two-phase  circuit.  The 
armature,  Fig.  143,  is  composed  of  two  separate  cores  assem- 
bled side  by  side.  The  secondary  winding,  polyodontal  as 
usual,  is  common  to  the  two  cores.  The  transformer  and 
motor  functions  are  here  entirely  distinct,  for  each  half  of  the 
machine  acts  alternately  as  transformer  and  motor,  each  set  of 
fields  inducing  current  which  serves  for  motor  purposes  in  the 
other  half  of  the  machine.  There  is  no  rotary  field  in  the 
ordinary  sense  of  that  term,  since  there  is  no  physical  result- 
ant of  the  two  field  magnetizations,  nothing  but  the  alterna- 
tion of  transformer  and  motor  functions  that  is  a  characteristic 
of  all  polyphase  induction  motors. 


246 


ELECTRIC    TRANSMISSION   OF  POWER. 


These  motors  are  generally  used  in  connection  with  con- 
densers to  improve  the  power  factor,  and  to  facilitate  this 
practice  are  usually  wound  for  500  volts. 

The  "  monocyclic  "  induction  motor  is  in  structure  similar  to 
ordinary  polyphase  induction  motors,  but  differs  from  them  in 
function,  in  that  under  normal  running  conditions  the  energy 
received  by  it  is  essentially  monophase.  Its  principle  can  be 
best  seen  by  referring  to  Fig.  130.  Suppose  that  instead  of 


FIG.  143. 

winding  the  A  and  B  poles  alike,  the  A  poles  are  proportioned 
so  as  to  supply  all  the  energy  required  by  the  motor,  while  the 
voltage  supplied  to  the  B  poles  is  sufficient  merely  for  magnet- 
izing purposes — /.  e.,  it  is  so  near  to  the  counter  E.  M.  F.  of 
the  revolving  armature  at  normal  speed  that  there  can  be  no 
material  transfer  of  energy.  The  B  poles  are  then  specialized 
as  field  poles,  while  the  A  poles  serve  in  lieu  of  brushes  to 
supply  the  energy.  The  magnetizing  current  for  the  B  poles 
is  furnished  by  a  separate  circuit  in  order  that  the  magnetiza- 
tion may  be  in  phase  with  the  armature  current.  The  E.  M.  F. 
of  this  circuit  is  thus  about  90°  ahead  of  that  of  the  power 
circuit,  while  the  currents  in  the  A  and  B  circuits  are  nearly 
in  phase.  Only  at  heavy  loads  is  there  any  material  transfer 
of  energy  from  B  to  the  armature.  The  object  of  this  rather 
singular  construction  is  to  confine  the  main  distribution  of 
energy  to  a  single  circuit,  the  small  magnetizing  current  being 
sent  over  a  subsidiary  wire  of  small  cross-section,  as  already, 


SYATCHRONOUS  AND  INDUCTION  MOTORS. 


247 


described.  The  motors  actually  used  on  monocyclic  circuits 
are  nearly  always  ordinary  three-phase  motors  in  the  mono- 
cyclic  connection. 

A  step  further  in  the  same  direction  of  simplicity,  but  inferior 
to  both  polyphase  and  heterophase  forms,  are  the  true  mono- 
phase induction  motors.  The  principle  of  these  motors  is 
shown  in  Fig.  144.  Here  there  is  but  one  set  of  poles  energized 


FIG.  144. 

by  the  circuit  A,  while  £,  <r,  d,  are  portions  of  the  armature  wind- 
ing, which  may  be  a  simple  squirrel  cage,  or  a  complex  bar 
winding  similar  to  those  used  in  polyphase  motors. 

If  A  be  supplied  with  an  alternating  current,  induced  currents 
will  be  produced  in  the  armature,  out  of  phase  with  the  field 
magnetization  and  symmetrical  with  respect  to  it,  so  that  no 
torque  is  produced. 

If,  however,  we  spin  the  armature  up  to  nearly  synchronous 
speed,  the  armature  currents  will  lag,  from  self-induction, 
behind  the  E.  M.  F.  set  up  by  the  field,  so  that  they  have  an 
angular  displacement  with  respect  to  the  field  at  a  time  when 
the  latter  is  still  active.  There  is  therefore  torque  between 
these  two  elements — ///  the  direction  of  the  initial  rotation. 
The  motor  will  thus  run,  when  once  started,  equally  well  in 
either  direction. 

In  every  motor  there  must  be  not  only  a  field  magnetization 
and  current  in  a  movable  conductor  substantially  in  phase 
with  each  other,  but  there  must  be  a  stable  angular  displace- 
ment between  the  two  in  order  to  ensure  continuous  torque. 
In  continuous  current  motors  this  displacement  is  secured  by 
the  position  of  the  brushes.  In  polyphase  induction  motors  it 
is  obtained  by  the  space  relation  of  the  sets  of  poles  combined 


248 


ELECTRIC   TRANSMISSION  OF  POWER. 


with  the  time  relation  of  the  two  or  more  currents.  In  the 
monocyclic  motor  the  currents  are  nearly  in  phase,  and  the 
condition  is  like  that  in  a  continuous  current  motor  with  the 
substitution  of  inductor  poles  for  brushes. 

In  the  monophase  motor  this  angular  displacement  is  due 
to  the  displacement  of  the  armature  currents  by  inductance. 
Hence  there  is  a  particular  value  of  the  inductance  correspond- 
ing to  the  best  condition  of  torque,  more  or  less  than  this 
being  highly  injurious. 

In  practice  monophase  induction  motors  are  built  in  very 
much  the  same  form  as  polyphase  motors,  and  for  the  same 
reason,  /.  ^.,  to  make  the  structure  good  as  a  transformer.  In 


FIG.  145. 

fact,  the  same  motor  structures  are  often  used  for  both  types. 
Fig.  145  shows  the  manner  of  winding  a  six-pole  monophase 
primary,  homologous  with  Figs.  137,  138.  A  monophase 
induction  motor  of  120  HP  by  Brown,  Boveri  &  Co.  is  shown 
in  Fig.  149.  Monophase  motors  are  not  used  to  any  consider- 
able extent  in  this  country,  and  abroad  their  use  is  generally 
confined  to  motors  much  smaller  than  the  example  shown. 

A    moment's  reflection  will  show  that  while  the  supply  of 
energy  to  a  polyphase  motor  is  substantially  continuous,  in 


SYNCHRONOUS  AND  INDUCl^ION  MOTORS.  249 

some  heterophase,  and  all  monophase,  motors  it  is  essentially 
intermittent,  so  that  these  latter  give  less  output  for  the  same 
structure.  The  dependence  of  the  torque  on  the  existence  and 
magnitude  of  the  armature  inductance,  too,  leads  to  low  power 
factors  and  great  difficulties  in  speed  regulation— never  too 
easy  in  induction  motors. 

As  we  have  already  seen,  polyphase  induction  motors  are 
self-starting.  To  give  a  monophase  motor  a  definite  starting 
torque  is  not  a  difficult  matter.  In  normal  monophase  work- 
ing there  is  no  initial  torque,  and  to  obtain  it  some  form  of 
heterophase  connection  is  generally  used.  The  commonest 
method  is  to  employ  a  set  of  subsidiary  windings  to  produce  tem- 
porary motor  poles,  with  which  the  current  produced  by  the 
matn  or  inductor  poles  can  react  as  in  the  polyphase  systems. 
When  the  motor  reaches  speed  this  subsidiary  winding  is  cut 
out  or  thrown  in  series  with  the  main  winding,  and  the  motor 
thereafter  runs  as  already  described.  The  starting  windings 
are  usually  energized  by  a  current  out  of  phase  with  the  main 
current,  but  derived  from  it  with  a  phase  difference  produced 
by  a  difference  of  inductance. 

By  reason  of  inefficient  starting  and  the  disadvantage 
inherent  in  discontinuous  energy  supply,  monophase  motors  in 
this  country  have  made  very  little  headway.  A  few  small 
motors  have  from  time  to  time  gone  into  use  on  circuits  still 
adhering  to  the  old  frequency  of  i20~  to  130^,  and  a  few 
more  on  the  occasional  monophase  circuits  at  60  cycles. 
Now  and  then  the  use  of  such  motors  is  convenient,  but  there 
is  no  reason  for  using  them  on  circuits  for  power  supply  only, 
and  they  are  apt  to  cause  trouble  if  used  on  lighting  circuits 
on  account  of  the  large  starting  currents.  When  occasion 
requires  their  employment  in  sizes  larger  than  a  horsepower 
or  two,  it  is  advisable  to  follow  the  European  practice  of  start- 
ing them  on  a  loose  pulley  and  then  taking  up  the  load  with  a 
clutch. 

One  of  the  recent  American  contributions  to  the  list  of 
monophase  motors  is  somewhat  out  of  the  ordinary  in  that  it 
starts  as  an  induction  motor  by  the  aid  of  a  commutator. 
This  is  the  Wagner  motor  shown  in  Fig.  146.  In  its  general 
construction  it  is  a  pure  monophase  motor  with  an  armature 
winding  the  coils  of  which  are  at  one  end  connected  with  a 


250 


ELECTRIC    TRANSMISSION   OF  POWER. 


commutator.  This  has  bearing  on  it  a  pair  of  brushes  which 
close  upon  themselves  those  armature  coils  which  are  in  such 
angular  relation  with  the  field  magnetization  as  to  give  a 
strong  motor  reaction  with  it.  By  thus  keeping  in  action 
only  coils  giving  an  efficient  torque  in  one  direction,  the 
necessary  directed  torque  at  starting  is  secured,  and  when  the 


FIG.  146. 

motor  reaches  a  predetermined  speed  a  compact  little  centrif- 
ugal governor  throws  over  a  short-circuiting  ring,  converting 
the  motor  into  an  ordinary  monophase  induction  motor.  It 
is  possible  to  start  under  load  with  this  device  by  drawing 
heavily  on  the  mains  for  current,  but  in  any  except  the 
smallest  sizes  it  is  better  to  start  light,  as  with  other  mono- 
phase motors. 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  251 

Although  monophase  motors  as  a  class  start  at  a  great  dis- 
advantage compared  with  polyphase  motors,  they  can  be  made 
to  give  pretty  good  results  at  load  by  extraordinary  care  in 
designing.  Fig.  147  shows  the  curves  obtained  from  a  certain 
Brown  motor  by  Professor  Arno.  The  motor  was  nominally 
of  15  HP,  but  was  evidently  overrated  at  that  load.  Never- 
theless within  a  certain  range  of  load  the  performance  of  this 
motor  compares  well  with  that  of  the  best  polyphase  motors 
of  similar  size.  This  motor  had  an  extremely  small  air  gap, 
and  shows  doubtless  a  record  performance  in  several  re- 


FIG.  147. 

spects,  but  it  proves  that  barring  the  matter  of  starting  it 
would  be  possible  to  turn  out  a  pretty  useful  machine  of  the 
monophase  type  if  anybody  desired  it,  although  it  is  certain 
that  at  anything  like  equal  cost  of  construction  the  polyphase 
motor  must  retain  the  advantage.  Fig.  148  shows  the  charac- 
teristic curves  of  a  recent  4  HP  Wagner  motor  which  gave  a 
highly  creditable  performance  for  a  monophase  motor  of  so 
small  size.  It  comes  much  nearer  representing  real  commer- 
cial conditions  than  the  curves  of  Fig.  147,  and,  as  we  shall 
presently  see,  does  not  make  a  bad  showing  as  compared  with 
the  polyphase  motors  ordinarily  found  upon  the  market. 
Speed  regulation,  never  any  too  easy  in  induction  motors,  is 
almost  out  of  the  question  in  the  monophase  form.  Still, 
within  its  limitations  it  has  its  uses. 

The  practical  properties  of  good  modern  induction  motors 


25 2  ELECTX1C    TRANSMISSION   OF  POWER. 

are  strikingly  similar  to  those  of  shunt-wound  or  separately 
excited  continuous  current  motors. 

For  the  same  output,  the  induction  motor  generally  has  the 
advantage  in  weight,  owing  to  the  fine  quality  of  iron  which  has 
to  be  employed,  but  its  laminated  structure  and  rather  com- 
plicated primary  winding  make  it  fully  as  expensive  to  build,  in 
spite  of  the  absence  of  a  commutator. 

In  point  of  commercial  efficiency  there  is  but  little  differ- 
ence. It  is  not  difficult  to  build  an  induction  motor  which  is 
fully  up  to  the  average  efficiency  of  other  motors  of  similar 
output  and  speed.  And  what  is  of  greater  importance,  the 
question  of  sparking  being  eliminated,  the  point  of  maximum 


efficiency  can  quite  easily  be  brought  somewhere  near  the  aver- 
age load.  It  must  be  remembered  that  here,  as  elsewhere, 
the  last  few  per  cent,  of  efficiency  are  somewhat  costly,  and  not 
always  found  in  the  rank  and  file  of  commercial  machines. 

The  weak  point  of  commercial  induction  motors  is  apt  to  be 
the  power  factor.  Of  course  low  power  factor  means  demand 
for  current  quite  out  of  proportion  to  the  output,  and  hence 
greater  loss  in  the  lines  and  greater  station  capacity.  In 
addition,  a  heavy  lagging  current  makes  regulation  of  voltage 
on  the  system  anything  but  easy. 


SYNCHRONOUS  AND   INDUCTION   MOTORS.  253 

Now,  it  is  perfectly  feasible  to  build  induction  motors  with 
power  factors  so  high  as  to  avoid  these  practical  difficulties 
almost  entirely.  But  this  re€ult  Is  somewhat  expensive, 
whether  reached  \sy  finesse  in  design,  or  by  the  addition  of  con- 
densers, and  it  is  therefore  not  always  attained. 

Slow  speed  induction  motors,  large  and  small,  are  subject 
to  bad  power  factors,  and  so  in  fact  are  all  induction  motors 
having  many  poles.  The  best  results,  however,  are  very  good 
indeed.  A  power  factor  of  .9  or  thereabouts  at  normal  load 
is  quite  unobjectionable  in  practice,  and  this  figure  can  be 
reached  or  closely  approximated  by  careful  design. 


FIG.  149. 

In  point  of  etnciency  there  is  little  difficulty  in  reaching 
satisfactory  figures.  The  actual  properties  of  polyphase  induc- 
tion motors  can  be  best  appreciated  by  the  examination  of 
their  characteristic  curves,  showing  the  variations  of  efficiency, 
power  factor,  and  speed  under  varying  loads.  Fig.  150  shows 
these  curves  for  a  75  HP  three-phase  motor  built  by  the  General 
Electric  Company.  It  is  a  6o~  motor,  intended  for  severe  ser- 
vice, and  hence  is  arranged  to  carry  considerable  overload  at  a 
good  efficiency.  The  fall  in  speed  from  no  load  to  full  load  is 


ELECTRIC    TRANSMISSION  OF  POWER. 

but  3  per  cent,  and  the  starting  torque  is  80  per  cent  greater 
than  full  running  torque,  with  an  expenditure  of  current  closely 
proportional  to  the  torque.  The  commercial  efficiency  reaches 
•91.1  per  cent. ,  and  the  power  factor  84. 3  per  cent. ,  which  is  not 
bad  for  so  large  a  motor  intended  for  considerable  overloads. 
Fig.  151  shows  the  characteristics  of  a  Westinghouse  two- 
phase  induction  motor  of  50  HP  for  25 ~.  Its  properties,  as 
might  be  expected  of  a  well-designed  motor  for  so  low  a  fre- 
quency, are  admirable,  particularly  the  great  efficiency  at 
small  loads. 


10   20   30   40   50   60   70   80   iK)   100  110  120  BUF9 
FIG.  150. 

Fig.  152  shows  the  properties  of  a  polyphase  motor  of  20 
HP  at  130^,  used  with  Stanley  condensers  to  keep  down  the 
results  of  the  inductance  encountered  at  so  high  a  frequency. 
The  effect  of  this  device,  particularly  at  moderate  loads,  is 
very  striking  indeed.  Without  condensers  one  could  not 
obtain  such  a  power  factor  even  at  full  load.  While  the  con- 
denser does  not  perfectly  compensate  for  inductance,  it  does 
so  sufficiently  well  for  all  practical  purposes.  In  other  prop- 
erties the  motor  is  not  so  especially  remarkable. 

These  curves  are  from  the  manufacturers'  tests,  and  the 
author  believes  them  to  be  entirely  trustworthy,  although  they 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


255 


probably  represent  good  results.  Better  curves  than  these  are 
occasionally  obtained,  generally  for  some  individual  reason. 
Now  and  then  a  "  freak  "  motor  is  produced,  with  enormously 
high  efficiency  or  power  factor,  like  a  certain  5  HP  three-phase 
motor  designed  and  tested  by  the  author,  which  gave  at  full 
load  a  power  factor  of  .94. 

On  the  other  hand,  it  is  unfortunately  true  that  many  com- 
mercial induction  motors  are  not  as  good  in  point  of  efficiency 
and  power  factor  as  they  ought  to  be.  A  series  of  tests  of 


60  H.P.  [TWO  PHASE  JTESLA  MOTOR 
22O  VOLTS-3OOO  ALTS.-75O  R.P.M. 


MAXIMUM  TiORQU 
IMES  FULL  LOAD 


40          50          60 
OUTPUT-BRAKE  H.P. 


FIG.  151. 

induction  motors  under  the  direction  of  Professor  D.  C.  Jack- 
son has  recently  been  published,  which  gives  data  so  instructive 
and  impartial  as  to  be  well  worth  reproduction  here.  The 
motors  tested  were,  except  fora  10  HP  Westinghouse  two-phase, 
all  of  5  HP  nominal  capacity,  and  by  the  following  makers: 
Westinghouse,  Fort  Wayne  Electric  Corporation  (synchronous 
self-starting  monophase),  Stanley,  Allegemeine  Electricitats 
Gesellschaft,  General  Electric  Company.  In  addition,  results 
of  tests  on  Oerlikon  and  Brown  motors  are  included  in  the  re- 
sults. Fig.  153  shows  the  efficiency  curves  and  regulation  of 


256  ELECTRIC    TRANSMISSION   OF  POWER. 

the  several  machines,  and  Table  I  on  page  258  gives  a  gen- 
eral view  of  their  respective  properties. 

Looking  over  these  results,  Nos.  3,  5,  and  8  are  decidedly 
the  best  of  the  lot.  Of  these  No.  3  is  possessed  of  a  fairly  high 
and  very  uniform  power  factor,  but  rather  moderate  efficiency. 
It  starts  well,  and  with  a  moderate  current  has  sufficient 
margin  of  capacity  for  all  ordinary  work,  but  its  speed  falls 
considerably  under  load.  No.  5  has  extraordinary  efficiency  at 
all  loads,  starts  admirably,  and  can  carry  a  tremendous  over- 
load— more  than  double  its  rated  capacity.  Moreover,  it 
regulates  very  closely.  The  power  factor,  however,  is  so  bad 
as  to  be  a  curiosity,  having  apparently  been  sacrificed  to 


10        12        11        10        IX        20        'M 

MECHANICAL  HORSE  POWER 
FIG.   152. 

obtain  great  maximum  output,  which  is  for  many  purposes  use- 
less. No.  8  is  afar  better  all-around  machine  than  any  of  the 
others,  has  a  good  maximum  efficiency  at  a  little  below  full 
load,  and  an  excellent  power  factor.  Professor  Jackson  notes 
that  since,  at  an  output  of  3^  HP,  No.  3  has  an  elriciency  of 
75.5,  and  a  power  factor  of  83^,  while  No.  5  shows  respec- 
tively 85  and  59,  the  station  capacity  for  the  latter  must  be  con- 
siderably greater  than  for  the  former.  That  is,  the  apparent 
efficiency  of  No.  3,  which  determines  the  necessary  station 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


257 


capacity,  is  64  per  cent.,  while  that  of  No.  5  is  50  per  cent., 
Hence  to  supply  one  brake  HP  with  No.  5  motors,  there  must 
be  a  station  capacity  of  2  HP,  while  with  No.  3  motors  1.56  HP 
is  sufficient.  But  with  No.  8  the  efficiency  is  about  .83,  and  the 
power  factor  about  .80,  giving  an  apparent  efficiency  of  .66, 
which  is  better  than  either  No.  3  or  No.  5.  Motors  like 
No.  3  are  excellent  for  the  power  station,  but  hard  on  the 
customer,  while  No.  5  is  admirable  for  the  customer,  but  bad 
for  the  station.  No.  8  is  fair  to  both  parties. 

Most  of  the  motors  shown  start  quite  well  enough  for  ordi- 
nary purposes.  Neither  heavy  starting  torque  nor  ability  to 
carry  large  overloads  is  needed  in  ordinary  motor  work. 
Large  torque  per  ampere  is,  however,  desirable.  It  is  best 


MAXIMUM  LOADS 
EFFICIENCIESAND  REGULATION 


1000 


2000  3000      -       1000 

FIG.  153. 


5000 


secured  by  using  at  starting  a  non-inductive  resistance  in  the 
secondary  circuit.  The  effect  of  this  resistance  is  as  follows: 
It  reduces  the  current  drawn  from  the  mains  so  that  the 
motor  will  not  seriously  disturb  the  voltage  on  the  lines  at 
starting;  by  diminishing  the  current  flowing  in  the  armature 
it  limits  the  armature  reaction  so  that  it  may  not  beat  back  the 
field  so  as  to  interfere  with  proper  starting,  nor  distort  it  so 
as  to  produce  dead  points;  and,  finally,  it  largely  increases  the 
torque  per  ampere,  which  greatly  aids  in  starting  under  load. 


258 


ELECTRIC   TRANSMISSION  OF  POWER. 


The  function  first  mentioned  is  very  important  where  lights 
and  motors  are  to  be  operated,  since  if  a  motor  is  capable  of 
starting  under  heavy  load  it  is  likely  to  take  at  starting  a 
pretty  large  current,  which  may  pull  down  the  voltage  in  the 
neighborhood  merely  in  virtue  of  ohmic  drop.  Besides,  the 
power  factor  of  an  induction  motor  at  starting  is  only  about 
.7,  so  that  the  heavy  current  lags  severely  and  still  further 
interferes  with  proper  regulation. 

The  heavy  lagging  current  set  up  in  the  armature  is  likely 
to  distort  the  field  seriously,  sometimes  so  much  as  to  block 
the  starting  of  the  motor,  sometimes  merely  producing  dead 
points,  i.  e.,  points  of  no  torque,  or  greatly  weakening  the 

TABLE  I. 

COMPARATIVE   QUALITIES. 


5 

10 

4-5 
5 


5 
5 
6.26 


0-55 
10 
4  5 
5 


TORQUE/  in  % 
OF  FULL  LOAD. 


140 

100 

104 

i86.S 


131 

45 
QO 
136 
138. 
142 
232 


77 

S 

3Q-4 
86.6 
1.37 


1 


9»779 
7 

3-7 
4 

l:l 


EFFICIENCIES—  %. 


582 


77-8 

83.8  54-5  72 


55-2 

54 

S2 

5  62.2 


75.28 


8655 


70.6 
5 

7i.9 
77-2 


80.2 


80.5 
78. q 
79-5 
87.7 

Sl.Q 

8.4 


83. 


POWER  FACTORS—  %. 


6.'4 


78-5 
70 


37.5  50      62.3 
67.3 

83-383-7 
44-762-573.7 
27-545-5  S9-8 

59.i69. 
.3  69  4)80.3 

7i-5  " 


64 

84 

80.3 

70-3 

73-4 

85.2 

87.1 


v NOTE.— 6,  7  and  8  were  not  run  up  to  maximum  load,  on  test, 
torque  in  certain  positions  of  the  armature.  The  introduc- 
tion of  resistance  in  the  secondary  circuit  both  diminishes  the 
current  and  its  angle  of  lag,  and  thus  keeps  down  the  arma- 
ture reaction.  In  some  motors  the  reluctance  of  the  magnetic 
circuits  is  sensibly  the  same  in  all  angular  positions  of  the 
armature,  so  that  there  are  no  points  of  noticeably  weak 
torque  either  with  or  without  a  starting  resistance.  But 
some  motors  otherwise  excellent  have  sufficient  variations  of 
reluctance  to  produce  bad  dead  points  when  the  armature 
reaction  is  severe,  while  these  nearly  or  quite  disappear  by 
adding  resistance  in  the  secondary  circuits. 


SYNCHRONOUS  AND  INDUCTION  MOTORS.  259, 

The  use  of  resistance  in  the  secondary  at  starting  obviously 
throws  forward  the  phase  of  the  secondary  current  so  that  it 
is  in  better  relation  to  the  field  magnetization,  and  hence 
although  the  numerical  value  of  the  current  is  reduced,  its- 
effective  component  is  increased.  The  considerations  which. 
affect  the  relations  between  torque  and  current  in  the  arma- 
tures of  induction  motors  are  in  reality  quite  simple.  The 
absolute  value  of  the  current,  other  things  being  equal,  is 
determined  by  the  armature  impedance,  and  is  the  same  for 
the  same  impedance  whatever  the  relation  between  the  react- 
ance and  resistance  components  of  that  impedence.  The 
ratio  between  these  components,  however,  determines  the 
phase  angle  of  the  armature  current,  so  that  for  a  given  value 
of  the  current  the  torque  depends  on  the  ratio  between  resist- 
ance and  reactance  in  the  armature. 

By  lessening  either  the  resistance  or  reactance  of  the  arma- 
ture a  motor  is  obtained  in  which  a  very  large  current  flows- 
at  starting,  but  reducing  the  impedance  by  cutting  down 
reactance  gives  the  resulting  current  a  better  phase  angle 
than  that  obtained  by  reducing  resistance  alone.  For  a  given 
motor  the  maximum  torque  is  obtained  when  the  ratio  of 
resistance  and  reactance  is  unity,  i.  e.,  when 


Now  one  can  cut  down  the  resistance  by  increasing  the 
allowance  of  armature  copper,  and  can  diminish  the  reactance 
by  subdividing  the  winding  so  that  there  shall  be  many  slots 
in  the  armature,  and  the  minimum  possible  number  of  turns 
per  slot.  Also  the  better  the  mutual  induction  between  field 
and  armature  the  less  the  reactance  of  either  member  is  likely 
to  be,  so  that  by  close  attention  to  design  it  is  possible 
greatly  to  reduce  the  armature  reactance.  In  commercial 
motors  the  relation  between  resistance  and  reactance  in  the 
armature  is  generally  from 


/  =  3^?  to      = 

Hence  when  large  torque  per  ampere  is  desired  the  simplest 
thing  to  do  is  to  insert  non-inductive  resistance  in  the  second- 
ary, and  when 


260  ELECTRIC    TRANSMISSION  OF  POWER. 

the   given   motor  will   be  at  its  best  with  respect  to  starting 

torque.     With 

I  =  R 

the  largest  torque  will  be  obtained  when  both  are  as  small  as 
possible.  Hence  if  very  great  starting  torque  is  desired  the 
motor  should  be  designed  with  very  low  armature  resistance 
and  reactance. 

The  slip  of  the  motor  below  synchronous  speed  depends 
upon  the  armature  resistance  in  induction  motors,  just  as  in 
continuous  current  motors  the  slip  below  the  speed  at  which 
the  armature  would  give  the  impressed  E.  M.  F.  is  determined 
by  armature  resistance.  In  each  case  the  slip  measures  the 
percentage  of  energy  lost  in  the  armature,  so  that  if  an  induc- 
tion motor,  for  example,  runs  loaded  at  5  percent,  slip  the  loss 
of  efficiency  in  the  armature  is  5  per  cent. 

Commercial"  induction  motors  vary  widely  in  slip — from  as 
little  as  i  per  cent,  to  8  or  10  per  cent.,  according  to  design. 
It  must  not  for  a  moment  be  supposed,  however,  that  small 
slip  implies  high  efficiency  of  the  motor.  One  can  put, 
in  designing  a  motor,  most  of  the  loss  into  the  armature  or 
into  the  field,  as  one  pleases,  and  it  is  pretty  safe  to  say  that 
if  there  is  remarkably  little  in  the  armature  there  will  be  an 
unusual  amount  in  the  field,  unless  cost  is  utterly  disregarded. 
Proba'bly  the  best  all-around  results  can  be  obtained  by  divid- 
ing the  permissible  loss  nearly  equally  between  armature  and 
field. 

There  is  a  very  simple  relation  between  the  static  and  run- 
ning torques  of  an  'induction  motor,  the  static  and  running 
currents,  and  the  slip,  as  follows: 

T  r*a 

—  S 

T  TT>' 

*  a  us 

In  this  equation  T  is  the  static  torque,  C  the  static  current, 
Ta  and  C9  torque  and  current  of  the  slip  s,  and  s  that  slip 
expressed  as  a  percentage.  As  an  example  of  the  application 
of  this  formula,  suppose  the  full  load  current  of  a  certain 
motor  is  60  amperes  per  phase,  the  current  with  the  armature 
at  rest  400  amperes,  and  the  slip  at  full  load  is  5  per  cent. 
Then 

T  160000 

=   .05    X  -   =  2.22, 

/  8  3600 


SYNCHRONOUS  AND   INDUCTION  MOTORS. 


261 


/.  <?.,  the  static  torque  will  be  2.22  times  the  full  load  running 
torque.  Of  course  if  a  motor  is  to  have  a  powerful  starting 
torque  it  must  take  a  pretty  heavy  current,  but  the  extra 
resistance  at  starting  helps  very  materially  in  keeping  the 
current  within  bounds.  An  adjustable  secondary  resistance 
makes  it  easy  to  bring  to  speed  any  load  that  the  motor  will 
carry  continuously,  without  demanding  excessive  current. 

As  to  overload,  an  ability  to  carry  25  per  cent,  more 
than  the  rated  capacity  is  ample,  save  in  rare  cases,  and 
greater  margin  than  this  usually  means  some  sacrifice  in  effi- 


7          8          9         1O        11 

Speed  ID  100  r-p.m. 
FIG.   154. 

ciency  or  power  factor  at  normal  loads.  For  most  work  an 
efficiency  curve  like  that  of  No.  8  is  preferable  to  one  like  that 
of  No.  5.  When  great  margin  of  capacity  is  needed,  it  is  best 
to  use  a  motor  deliberately  adjusted  to  such  use,  and  not  to 
-expect  it  of  a  motor  properly  designed  for  ordinary  service. 

The  speed  of  induction  motors  is  best  regulated  by  inserting 
a  non-inductive  resistance  in  the  secondary  circuit.  Under 
these  circumstances  the  motor  can  be  made  to  run  at  con- 
stant torqus  over  a  very  wide  range  of  speeds  by  varying  the 


262  ELECTRIC    TRANSMISSION   OF  POWER. 

resistance,  just  as  one  would  regulate  a  street-car  motor. 
Fig.  154  shows  the  variation  in  speed,  current,  and  power 
factor  in  a  15  HP  three-phase  motor  fitted  with  rheostatic 
control.  The  speed  was  varied  at  constant  torque  from  about 
1400  r.p.m.  down  to  150  r.p.m.  Curve  B  shows  the  variation 
cf  the  power  factor,  in  this  case  high  at  all  speeds,  and  curve 
C  shows  the  slight  variation  in  input.  Operated  in  this  way, 
the  motor  behaved  almost  exactly  like  a  series-wound  direct 
current  motor  with  rheostatic  control.  Such  a  rheostat  is 
used  in  operating  hoists  and  the  like  with  induction  motors. 
Regulation  by  varying  the  primary  voltage  is  highly  unsatis- 
factory, since  the  torque  falls  off  nearly  in  proportion  to  the 
square  of  the  voltage,  so  that  at  low  speeds  the  output  is 
enormously  reduced.  Regulation  by  any  method  involving 
resistance  is  of  course  inefficient,  not  materially  more  so,  how- 
ever, than  in  the  case  of  continuous  current  motors.  It  should 
be  understood  that  all  these  remarks  concerning  torque, 
regulation,  and  the  like  apply  to  polyphase  induction  motors 
and  do  not  hold  true  in  general  of  monophase  motors. 

The  weak  point  of  induction  motor  practice  is  in  the  heavy 
inductance  likely  to  be  encountered  unless  motors  with  first- 
class  power  factors  are  used.  It  is  depressing  to  find  the 
current  capacity  of  your  generator  exhausted  long  before  it 
has  reached  its  rated  output  in  kilowatts,  and  if  the  motor 
.service  is  part  of  a  general  system,  the  effect  of  a  bad  power 
factor  on  regulation  is  disastrous. 

!  With  generators  of  moderate  inductance  and  good  motors, 
'general  distribution  by  polyphase  currents  gives  admirable 
results.  The  station  manager  should  see  to  it  that  his  motors 
are  not  of  excessive  size  for  their  work,  and  are  good  in  the 
matter  of  power  factor.  A  few  motors  for  very  variable  loads 
can  be  handled  readily  enough,  but  no  motor  with  a  bad 
power  factor  should  be  tolerated  simply  because  it  is  cheap. 
Power  factors  of  at  least  .85  at  full  load,  and  .80  at  two-thirds 
load,  are  quite  obtainable  except  in  case  of  some  special  motors, 
and  should  be  insisted  upon  rigorously. 

One  polyphase  station  operating  more  than  fifty  induction 
motors  showed,  when  tested  by  the  author,  about  .65  as  aver- 
age power  factor  when  carrying  all  the  motors.  Rigorous 
inspection  of  the  motors  installed  would  have  raised  this 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  263 

figure  to  .75,  although  the  existing  power  factor  actually  gave 
no  trouble,  there  being  ample  generator  capacity. 

So  much  for  polyphase  induction  motors.  Heterophase  and 
monophase  induction  motors  generally  fail  to  give  so  uni- 
formly good  results.  Occasional  extraordinary  results  have 
been  reported  from  the  latter  class,  but  in  the  author's  opinion 
they  concern  motors  which  belong  to  the  "freak"  class 
alluded  to,  and  cannot  be  expected  in  commercial  practice. 
Monophase  motors  are  usually  weak  in  power  factor,  and  start 
badly,  most  of  those  in  use  abroad  being  started  without  load. 
Even  so,  the  starting  current  is  large,  as  may  be  safely  con- 
cluded from  the  discreet  silence  preserved  on  this  topic  in 
all  descriptions  of  monophase  motor  installations.  In  general 
power  transmission  work  the  incandescent  lamp  and  the  in- 
duction motor  are  the  chief  factors.  Synchronous  motors  are 
valuable  in  their  proper  place,  and  arc  lighting  and  continu- 
ous current  work  are  sometimes  relatively  important.  The 
alternating  current  systems  are  now  far  enough  developed  to 
be  entirely  workable  and  trustworthy  for  incandescents  and 
motors.  The  alternating  arc  lamp  is,  however,  not  quite  in 
condition  to  replace  the  continuous  current  arcs  for  all  pur- 
poses and  under  all  circumstances,  and  for  work  specially 
suited  to  continuous  currents  reliance  has  at  present  to  be 
placed  in  various  current  reorganizing  devices,  which  are,  so 
far,  of  rather  indeterminate  ultimate  value.  Whether  they 
are  to  have  a  large  permanent  place  in  the  art,  or  whether 
their  sphere  will  gradually  be  much  contracted,  is  uncertain, 
although  the  author  inclines  to  the  latter  opinion.  At  all 
events  it  is  sufficiently  clear  that  the  main  body  of  power 
transmission  will  have  to  depend  on  alternating  currents,  at 
least  for  a  long  while  to  come. 

Even  if  continuous  current  should  be  obtained  somewhat 
directly  from  coal  in  the  near  or  far  future,  the  result  would  be 
not  to  increase  power  transmission  by  continuous  currents, 
but  to  render  the  transportation  of  coal  by  far  the  cheapest 
method  of  transmitting  energy. 

The  relative  importance  of  polyphase,  heterophase  and 
monophase  systems  is  a  question  often  raised.  The  present 
indications  are  that  the  polyphase  systems,  in  virtue  of  in- 
creased output  of  generators,  possible  economy  in  coppc"  c.nd 


264  ELECTRIC    TRANSMISSION  OF  POWER. 

general  convenience,  have  come  to  stay.  The  monophase 
motor  problem  has  not  yet  been  satisfactorily  solved  in  any 
general  way,  and  until  it  has  been  solved,  the  monophase  sys- 
tem must  remain  subordinate,  like  the  heterophase  systems, 
which  are  special  rather  than  general  in  their  applicability. 
The  much  mooted  question  of  frequency  will  be  referred  to  in 
its  different  bearings  in  connection  with  other  topics.  The 
frequencies  once  common,  i2o~  to  135 ~,  are  rapidly  passing 
out  of  use  for  all  important  work.  They  are  inconveniently 
high  for  long  lines  by  reason  of  inductance,  are  troublesome 
for  large  units,  lead  to  high  inductance  in  the  system,  and 
have  for  their  only  compensating  advantage,  lessened  cost  of 
transformers.  Both  here  and  abroad  lower  frequencies  have' 
come  into  use.  In  this  country  6o~  to  65  ~  seems  to  be  the 
favorite  range,  except  for  work  with  rotary  converters,  when 
25  ~~  to  35  ~  is  usual.  Both  these  last  are  too  low  for 
general  practice,  since  the  cost  of  transformers  is  greatly 
increased;  the  former  is  unsuitable  for  incandescent  service, 
unless  with  extremely  low  voltage  lamps,  and  both  are  un- 
suitable for  alternating  arcs.  It  is  now  pretty  generally 
recognized  for  the  above  reason  that  the  adoption  of  so  low  a 
frequency  as  25 ~  in  the  great  Niagara  plant  was  an  error  of 
judgment,  perhaps  brought  about  by  an  overestimate  of  the 
importance  of  rotary  converters  in  general  distribution. 

On  the  other  hand,  abroad  a  compromise  frequency  of  40^ 
to  5o~  is  in  general  use.  In  the  author's  opinion  there  are 
very  few  cases  in  which  lower  frequencies  than  these  are 
desirable,  and  none  in  which  less  than  30^  should  be  toler- 
ated; 5o~  to  6o~  meets  general  requirements  admirably, 
and  only  in  rare  cases  is  the  use  of  rotary  converters  of  suffi- 
cient importance  to  call  for  a  lower  frequency. 

In  connection  with  this  topic  we  may  consider  a  verbose 
controversy  which  has  raged  of  late,  respecting  the  advantages 
of  certain  irregular  forms  of  alternating  current  waves  vs.  a 
true  sine  wave.  The  facts  in  a  nutshell  are  as  follows:  Cer- 
tain complex  current  waves,  whose  irregularity  is  due  to  the 
presence  of  harmonics  of  higher  frequency,  have  been  found 
to  give  slightly  better  efficiency  in  transformers  than  sine 
waves  of  the  same  nominal  frequency.  Such  waves,  however, 
do  not  hold  their  form  under  varying  conditions  of  load,  and 


SYNCHRONOUS  AND   INDUCTION  MOTORS.  265 

by  reason  of  their  harmonics  of  higher  frequency  raise  the 
inductance  of  the  line  and  apparatus,  increase  the  probability  of 
resonance  on  the  line,  disorganize  all  attempts  to  balance  the 
inductance  of  the  system  by  condensers  or  synchronous  motors, 
and  finally  sometimes  interfere  with  the  proper  performance  of 
induction  motors.  The  use  of  such  wave  forms,  then,  is  likely 
to  lead  to  very  embarrassing  complications  in  a  power  trans- 
mission system,  and  their  sole  advantage  is  far  better  secured 
by  using  a  sine  wave  of  slightly  increased  frequency,  than  by 
interpolating  a  set  of  worse  than  useless  harmonics. 

It  is  needless  to  say  that  all  cases  of  power  transmission 
cannot  be  treated  alike — there  is  no  system  that  will  meet  all 
conditions  in  the  best  possible  manner.  The  best  results  will 
be  obtained  by  treating,  in  the  preliminary  investigation, 
each  problem  as  an  unique  and  independent  case  of  power 
transmission,  and  afterward  boiling  down  the  conclusions  to 
meet  practical  conditions.  Avoid,  when  you  can,  apparatus  of 
peculiar  sizes  and  speeds — remember  that  you  are  after  re- 
sults, not  electrical  curios.  See  to  it  that  what  is  done  is 
done  thoroughly,  and  for  general  guiding  principles  keep  your 
voltage  up  and  your  inductance  down,  and  watch  the  line. 


CHAPTER   VII 

CURRENT     REORGANIZERS. 

WHATEVER  method  may  be  employed  for  the  transmission 
of  power  in  any  given  case,  it  will  often  be  found  that  the 
current  delivered  at  the  receiving  station  is  not  of  the  charac- 
ter needed.  Sometimes  in  transmissions  for  special  purposes 
no  difficulty  will  be  met,  but  frequently,  especially  in  the 
transmission  of  power  for  general  distribution,  both  continu- 
ous and  alternating  currents  are  needed,  whereas  only  one  is 
at  hand.  For  all  electrolytic  operations,  for  railway  work  at 
present,  for  telegraphy,  and  sometimes  for  arc  lighting,  con- 
tinuous current  is  necessary,  while  alternating  current  is 
necessary  for  convenient  application  to  electric  furnaces, 
electric  welding,  electro-cautery  and  other  minor  purposes. 
So  whichever  kind  of  current  is  transmitted  the  other  must  be 
derived  from  it  for  certain  uses. 

All  devices  for  thus  changing  alternating  to  direct  currents, 
or  vice  versa,  with  or  without  accompanying  change  of  voltage, 
may  properly  be  called  current  reorganizes. 

Three  classes  of  such  apparatus  have  come  into  more  or 
less  use:  i.  Commutators;  2.  Motor  dynamos;  3.  Rotary 
converters.  These  classes  are  quite  distinct  from  each  other; 
each  has  advantages  and  faults  peculiar  to  itself,  and  all  three 
are  in  every-day  practical  use  to  a  greater  extent  than  would 
seem  probable  at  first  thought. 

We  have  already  looked  into  the  matter  of  commutation  in 
Chapter  I.,  and  have  seen  how  the  naturally  alternating  cur- 
rents in  a  continuous  current  dynamo  are  rectified  and 
smoothed.  Given,  then,  an  alternating  current  received  from 
a  distant  generator,  and  it  would  seem  an  easy  matter  to 
receive  this  current  upon  a  commutator  and  deliver  it  as  con- 
tinuous current.  In  point  of  fact  there  are  very  serious  diffi- 
culties in  this  apparently  simple  process. 

The  current  received  is  a  set  of  simple  alternations  shown 

266 


CURRENT  REORGANIZERS. 


267 


diagrammatically  in  Fig.  155.  The  figure  shows  three  complete 
periods.  Now  if  such  a  current  be  sent  into  a  simple  two-part 
commutator,  such  as  is  shown  in  Fig.  9,  Chapter  I.,  revolving 


FIG,  155 

at  such  a  speed  that  the  brushes  will  be  just  passing  from  one 
segment  to  the  other  every  time  the  current  received  changes 
direction,  the  result  will  be  a  rectified  current,  shown  in  Fig. 
156,  unidirectional,  it  is  true,  but  far  from  continuous.  Vari- 


FIG.  156. 

ous  modifications  of  this  simple  rectifying  apparatus  have  been 
and  are  in  extensive  use  for  supplying  current  to  the  field 
magnets  of  alternating  generators.  As  these  machines  are 
generally  multipolar,  the  two-part  commutator  has  been  modi- 
fied so  as  to  reverse  the  current  at  each  alternation.  Fig. 
157  shows  one  of  the  simple  forms  of  commutator  arranged 


FIG.  157. 

for  self-exciting  alternators.  It  consists  of  a  pair  of  metal 
cylinders  mounted  on  and  insulated  from  the  dynamo  shaft. 
Each  cylinder  is  cut  away  into  teeth,  and  the  two  are  mounted 
so  that  the  teeth  interlock  with  insulation  between  them. 
Each  pair  of  consecutive  teeth  acts  like  the  ordinary  two-part 
commutator,  and  there  are  of  course  a  pair  of  teeth  for  every 
pair  of  poles,  so  that  the  commutator  acts  at  each  alternation. 


268  ELECTRIC    TRANSMISSION  OF  POWER. 

The  resulting  rectified  current  is  then  led  around  the  field 
magnets  of  the  generator,  furnishing  either  the  whole  excita- 
tion, or  enough  to  compound  the  machine.  Such  a  current, 
however,  is  so  fluctuating  that  it  is  by  no  means  the  equivalent 
of  an  ordinary  continuous  current  for  magnetizing  purposes, 
hence  in  most  modern  machines  the  main  exciting  current  is 
furnished  by  a  small  exciting  dynamo,  driven  from  the  alter- 
nator shaft  or  by  separate  means,  while  the  rectified  current  is 
used  only  now  and  then  for  compounding. 

This  simple  current  reorganizer  is  very  successful  for  the 
purpose  described.  But  it  must  be  remembered  that  the 
amount  of  energy  concerned  is  trifling,  only  a  very  few  kilo- 
watts being  required  to  compound  even  the  largest  alternators. 
And  despite  this,  there  is  often  trouble  from  sparking,  such 
commutators  being  notoriously  hard  to  keep  in  good  order. 


FIG.  158. 

In  applying  the  same  process  to  rectifying  current  on  a  larger 
scale,  the  difficulties  from  sparking  are  very  serious,  in  fact 
generally  prohibitive.  And  the  worst  of  it  is  that  they  are 
inherent.  The  root  of  the  trouble  is  that  the  alternating^ 
current  on  a  line  used  for  general  purposes  cannot  be  kept 
accurately  in  step  with  the  motion  of  the  commutator.  To 
ensure  sparkless  commutation  the  conditions  must  be  as  shown 
in  Fig.  158. 

The  alternations  of  the  current  and  E.  M.  F.  are  shown  by  the 
solid  line,  while  the  brushes  at  the  moment  of  passing  from  one 
commutator  segment  to  the  next  must  take  the  position  b  b, 
with  respect  to  the  current.  That  is,  they  must  pass  from  one 
segment  to  the  next  at  the  moment  when  the  current,  just 
reversing,  is  practically  zero.  So  long  as  the  electromotive 
force  and  the  current  are  in  phase  with  each  other,  as  shown 
in  the  solid  line,  the  current  will  be  rectified  without  trouble- 
some sparking.  But  when  the  current  lags  behind  the  E. 
M.  F.,  as  shown  by  the  dotted  line  of  Fig.  158,  there  is  trouble 


CURRENT  REORGANIZERS.  269 

at  once.  The  brushes,  as  can  be  seen  from  the  dotted  pro- 
longations of  b  b,  must  break  a  considerable  current,  and  there 
is  certain  to  be  sparking.  Nor  can  any  point  be  found  for  the 
brushes  at  which  they  will  not  have  either  to  break  this  cur- 
rent or  to  pass  from  one  segment  to  the  next  while  there  is 
considerable  E.  M.  .F.  between  segments.  The  case  is  bad 
enough  in  a  compounding  commutator  having  a  position  fixed 
with  reference  to  the  E.  M.  F.  of  the  machine  and  dealing 
with  low  voltage  and  moderate  current.  The  inevitable  result 
is  sparking  that  can  be  only  mitigated  by  shifting  the  brushes, 
and  more  or  less  demoralization  of  the  compounding.  If  the 
current  be  received  from  a  distant  generator  on  a  commuta- 
tor driven  by  a  synchronous  motor,  the  condition  of  things 
is  much  worse.  When  the  current  lags  (or  leads),  not  only 
are  the  brushes  generally  thrown  out  of  step  with  it,  but  if 
there  is  a  sudden  change  of  phase  the  inertia  of  the  commuta- 
ting  apparatus  will  put  it  at  serious  variance  for  the  time  with 
both  current  and  E.  M.  F.  Add  to  this  the  disturbances  of 
phase  produced  by  armature  reaction  in  both  generator  and 
motor,  and  one  has  a  set  of  conditions  that  renders  sparking 
absolutely  certain.  The  most  that  can  be  done  to  help  matters 
is  to  employ  palliative  measures  to  delay  the  destruction  of  the 
commutator.  Aside  from  this  sparking,  it  is  nearly  out  of  the 
question  to  hold  the  voltage  of  the  rectified  current  steady  if 
the  phase  is  shifting,  as  it  often  is  likely  to  be. 

Incidentally  may  be  mentioned  the  fact  that  in  working  such 
a  commutating  apparatus,  as  in  rotary  converters,  the  direc- 
tion of  the  rectified  current  will  be  uncertain;  the  brush  which 
happens  to  be  on  a  positive  segment  when  the  brush  circuit  is 
closed,  will  stay  positive,  as  can  readily  be  seen  by  tracing 
out  the  rectifying  process  in  Fig.  157.  In  ordinary  com- 
pounding commutators  this  uncertainty  is  absent,  for  with 
the  brushes  in  a  fixed  position  the  positive  segments  will 
always  be  under  the  same  brush,  since  the  segments  are  fixed 
with  reference  to  the  armature  coils, 

No  small  amount  of  time  and  money  has  been  spent  in  try- 
ing to  work  out  a  successful  synchronizing  commutator.  The 
main  trouble  is,  of  course,  sparking,  and  the  exasperating  part 
of  the  problem  is  that  while  on  a  small  scale,  as  in  compound- 
ing alternators,  fair  results  can  be  obtained,  the  difficulties 


270  ELECTRIC    TRANSMISSION   OF  POWER. 

increase  enormously  with  the  output,  so  that  every  attempt  on 
a  scale  really  worthy  of  serious  consideration  has  ended  in 
discouragement  and  the  scrap  heap. 

The  great  usefulness  of  such  apparatus  if  of  reasonably  good 
qualities  has  made  this  field  of  experimentation  very  interesting, 
and  a  vast  amount  of  ingenuity  has  been  expended  in  elabor- 
ately devised  plans  for  reducing  sparking  and  minimizing  the 
evil  results  of  shifting  phase.  An  example  of  such  work,  of 
more  than  usual  merit,  was  shown  at  the  International  Congress 
of  1893  at  Chicago.  This  was  the  current  reorganizer  devised 
by  C.  Pollak,  for  use  in  connection  with  accumulator  installa- 
tions. It  was  intended  specifically  for  charging  accumulators, 
and  is  very  ingeniously  adapted  to  that  use.  Its  general 
appearance  is  shown  by  Fig.  159.  The  apparatus  consists  of  a 


FIG.  159. 

small  synchronous  motor  driving  a  commutator,  which  has,  in 
the  example  shown,  eight  segments  coupled  alternately  in 
parallel  so  as  to  produce  the  effect  of  Fig.  157.  The  Pollak 
commutator  is,  however,  peculiar  in  that  the  spaces  between 
segments  are  of  nearly  the  same  width  as  the  segments  them- 
selves, while  the  collecting  brushes  are  set  in  pairs,  so  that  by 
setting  one  of  each  pair  ahead  of,  or  behind  the  other,  the  ratio 
of  segment  width  to  space  width  can  be  changed.  In  charging 
accumulators  the  E.  M.  F.  of  the  charging  current  must  always, 
to  prevent  waste  of  energy,  exceed  the  counter  E.  M.  F.  of  the 
battery.  Hence  a  current  rectified  as  in  Figs.  156  and  157  can- 


CURRENT  REORGANIZERS.  271 

not  successfully  be  used.  The  arrangement  of  segments  just 
described  enables  the  brushes  to  be  so  set  that  contact  with  a 
segment  is  made  at  the  moment  when  the  rising  E.  M.  F.  of  the 
alternating  side  is  exactly  equal  to  the  counter  E.  M.  F.  of  the 
battery,  and  broken  when  the  falling  E.  M.  F.  reaches  the  same 
value.  Only  that  part  of  the  current  wave  of  which  the  E.  M.  F. 
exceeds  the  counter  E.  M.  F.  of  the  battery  is  used,  the  charging 
circuit  being  open  during  the  remainder  of  the  period.  When 
well  adjusted  and  used  on  a  circuit  nearly  non-inductive, 
the  machine  in  question  is  almost  sparkless  and  very  well 
adapted  for  the  particular  purpose  intended.  It  is  also  highly 
efficient,  the  only  losses  being  those  in  the  motor,  plus  brush 
friction.  The  total  amount  of  these  need  be  but  trifling, 
probably  less  than  5  per  cent,  of  the  output. 

But  such  apparatus  cannot  be  considered  as  a  general 
solution  of  the  problem,  for  while  quite  successful  for  an 
output  of  10  KW  or  so,  it  has  not  been  tested  in  large 
sizes,  nor  under  the  conditions  of  inductance  ordinarily  to  be 
expected  on  a  power  transmission  circuit.  For  the  reasons 
already  adduced  the  chances  for  success  are  not  good,  particu- 
larly since  all  questions  of  sparking  become  very  grave  when 
large  currents  must  be  dealt  with.  This  difficulty  is  well 
known  in  dynamo  working.  For  instance,  in  an  arc  machine 
there  may  be  frequent  recurrence  of  the  long,  wicked-looking, 
blue  sparks  familiar  to  every  dynamo  tender,  without  notice- 
able damage  to  the  commutator,  while  in  a  low  voltage 
generator  sparking  of  much  less  formidable  appearance  may 
put  the  machine  hors  du  combat  in  a  very  short  time. 

Bearing  all. this  in  mind,  it  is  but  natural  to  expect  that 
another  particular  solution  of  the  reorganizing  problem  might 
be  found  for  arc  lighting.  Here  the  irregularity  of  a  "  recti- 
fied" current  is  of  small  consequence,  while  the  small  amount 
of  current  cannot  cause  really  destructive  sparking  if  other  con- 
ditions are  fairly  favorable.  So  it  is  that  we  find  commutating 
apparatus  in  quite  successful  use  for  arc  lighting  in  connection 
with  alternating  stations.  The  form  of  apparatus  shown  in 
Fig.  160,  designed  by  Ferranti,  has  been  introduced  in 
several  British  stations  with  good  results.  The  commutating 
mechanism  is  cf  course  used  in  connection  with  a  "  constant 
current  "  transformer,  arranged  so  as  automatically  to  hold 


272 


ELECTRIC    TRANSMISSION   OF  POWER. 


the  current  closely  uniform  under  all  variations  of  load. 
Each  commutating  unit  supplies  two  separate  arc  circuits  of 
moderate  capacity — twelve  lights  in  each.  How  well  the  same 
device  works  at  several  times  the  E.  M.  F.  necessary  to  supply 
so  small  a  series,  is  now  beipg  demonstrated.  The  present 
tendency  in  central  station  practice  is  .to  employ  very  high 
voltages  for  arc  lighting — 50  to  100  or  125  lamps  in  series, 
thus  greatly  simplifying  both  the  station  equipment  and  the 
circuits.  The  rectifier  should  at  least  be  able  to  replace  the 
smaller  generators  now  in  use  and  such  machines  are  now  built 
for  as  many  as  sixty  lights.  This  is  probably  practical— in 


FIG.   T 60. 

fact  there  seems  to  be  no  good  reason  why  the  rectifier  should 
not  be  entirely  available  wherever  it  is  desirable  to  tvorlc 
series  arc  circuits  in  connection  with  a  transmission  plant. 
Although  not  in  use  sufficiently  long  to  enable  one  to  pass  a 
final  judgment,  the  machine  is  at  least  promising  and  worth 
careful  investigation.  There  seems  to  be  some  doubt  as  to  the 
successful  working  of  these  rectifiers  at  anything  except  rather 
low  frequencies,  30  to  40^  or  less,  but  such  a  difficulty  would 
appear  to  be  constructional  rather  than  inherent.  It  is  possi- 
ble that  the  alternating  arc  lamp  will  be  developed  far  enough 
to  render  continuous  current  arcs  entirely  unnecessary,  but  this 
remains  yet  to  be  proved,  although  the  inclosed  alternating  arc 
now  gives  highly  successful  results,  particularly  in  street 
lighting. 


CURRENT  REORGANIZERS.  273 

All  rectifying  commutators  now  in  practical  service  are  of 
very  limited  output — not  much  exceeding  10  to  20  KW,  an 
amount  merely  trivial  so  far  as  large  enterprises  are  concerned. 
For  railway  work  or  incandescent  lighting,  these  very  interest- 
ing machines  cannot  be  considered  in  the  race  at  present.  The 
general  problem  is  as  yet  unsolved  by  such  means,  useful  as 
they  may  be  for  special  purposes. 

The  current  delivered  by  rectifiers  is  in  a  measure  discon- 
tinuous, and,  hence,  is  not  the  full  equivalent  of  an  ordinary 
continuous  current.  The  Pollak  machine,  however,  which  is 
intended  to  be  used  with  a  somewhat  flat-topped  alternating 
current  wave,  has  been  successfully  employed  for  working 
motors  as  well  as  for  charging  accumulators.  It  is  not  impos- 
sible that  such  apparatus  may  yet  be  constructed  of  sufficient 
capacity  to  be  of  much  practical  service,  although  the  difficul- 
ties, as  has  already  been  pointed  out,  are  very  considerable, 
and  of  a  kind  very  hard  to  overcome.  Of  course,  polyphase 
currents  can  be  rectified  by  following  the  same  process  as  with 
monophase  current,  and  a  successful  apparatus  would  often 
find  some  place  in  transmission  plants. 

The  advantages  of  the  rectifying  commutator  are  simplicity, 
efficiency,  and  cheapness,  particularly  the  last.  The  working 
parts  are  a  small  synchronous  motor,  made  self-exciting  (and 
self-starting)  by  a  commutator,  and  one  or  more  rectifying 
commutators  driven  by  this  motor.  To  obtain  100  KW  out- 
put, it  is  not  necessary,  as  in  other  forms  of  current  reorgan- 
izers,  to  have  a  machine  nearly  as  large  and  costly  as  a 
100  KW  dynamo.  On  the  contrary,  a  oiae  or  two  horse 
power  motor  would  be  amply  powerful  to  drive  the  commu- 
tator, and  the  whole  affair  could  hardly  cost  a  quarter  as  much 
as  a  dynamo  of  the  same  capacity,  besides  being  of  greater 
efficiency,  particularly  at  partial  loads.  But  a  hundred  kilowatts 
is  far  beyond  the  output  of  any  rectifier  that  has  yet  been  put 
to  commercial  service,  and  even  a  hundred  kilowatts  is  but  a 
fraction  of  the  output  that  is  often  desirable  in  a  single  unit. 

On  the  other  hand  a  rectifier  must  require  at  least  the  same 
care  as  a  dynamo,  and  must  in  every  practical  case  be  employed 
in  connection  with  reducing  transformers  to  bring  the  alter- 
nating current  to  the  right  voltage.  The  regulation  too,  is 
somewhat  dubious,  since  compound  winding  is  out  of  the 


274  ELECTRIC  TRANSMISSION  OF  POWER. 

question.  And  the  current  is  at  best  disjointed,  likely  to 
produce  needless  hysteresis,  and  of  a  character  rather  hard  to 
measure  conveniently. 

To  sum  up,  the  rectifying  commutator,  while  quite  good 
enough  for  certain  particular  purposes,  has  so  far  given  no 
definite  promise  of  general  usefulness.  All  of  the  serious 
attempts  to  develop  it  on  a  considerable  scale  have  ended  in 
failure.  It  is  not  effectively  reversible,  so  that  the  task  of 
converting  continuous  to  alternating  currents  is  quite  beyond 
it.  While  the  cheapness,  lightness  and  efficiency  of  such 
apparatus  puts  it  in  these  particulars  far  ahead  of  any  other 
type  of  current  reorganizer,  the  verdict  of  experience  has  so 
far  been  adverse  in  spite  of  these  advantages,  and  engineers 
have  been  driven  to  other  and  more  cumbersome  devices. 

The  most  obvious  method  of  deriving  continuous  from  alter- 
nating currents,  is  to  employ  an  alternating  current  motor  in 
driving  a  continuous  current  dynamo.  The  two  machines 
may  be  connected  in  any  convenient  way,  by  belting,  clutching 
the  shafts  together,  or  by  putting  them  in  even  more  intimate 
connection  by  placing  two  armatures  on  the  same  shaft  or  two 
windings  on  the  same  core. 

The  procedure  first  mentioned  is  not  infrequent,  particularly 
when  a  transmission  of  power  plant  is  installed  in  connection 
with  an  existing  lighting  or  power  station.  A  synchronous 
motor  is  installed  in  place  of  the  previously  used  engines, 
belted  in  any  convenient  way  to  the  existing  generators, 
and  the  operation  of  the  station  goes  on  as  before.  Good 
examples  of  this  practice  may  be  found  at  Walla  Walla,  Wash, 
(monophase),  Springfield,  Mass,  (two-phase),  Taftville,  Conn., 
and  Sacramento,  Cal.  (three-phase).  Further  description  is 
unnecessary,  as  the  apparatus  is  in  no  way  out  of  the  ordinary, 
and  not  at  all  specialized  for  the  conversion  of  alternating  to 
continuous  currents. 

A  more  interesting  way  of  accomplishing  the  same  result  is 
by  the  use  of  a  twin  machine  comprising  motor  and  generator 
on  the  same  bed  plate,  or  even  on  the  same  shaft.  In  this  way 
the  reorganizing  apparatus  is  formed  into  a  compact  unit, 
convenient  to  install  and  to  operate,  and  possessing  an  effi- 
ciency higher  than  that  of  two  belted  machines,  by  the  belt 
losses  and  more  or  less  of  the  bearing  friction.  The  total 


CURRENT  REORGANJZERS.  275 

increase  of  efficiency  is  perhaps  5  per  cent.,  when  the  com- 
parison is  between  a  pair  of  coupled  machines  and  a  pair 
directly  belted,  or  more  if  the  belting  be  indirect.  Moreover 
the  motor  and  dynamo  parts  of  the  machine  can  each  be 
designed  so  as  to  give  the  best  efficiency  and  economy  of 
construction  possible  at  the  given  mutual  speed.  A  typical 
unit  of  this  class  is  shown  in  Fig.  161 — a  Siemens  continuous 


alternating  transformer.  The  motor  part  is  wound  for  2,000 
volts,  monophase,  and  the  dynamo  part,  of  the  well-known  Sie- 
mens internal  pole  type,  with  overhung  armature  and  brushes 
directly  on  the  windings,  delivers  continuous  current  at  150 
volts.  In  this  case  the  machine  has  three  bearings,  although 
in  many  cases  it  would  be  quite  possible  to  get  along  with 
two.  The  main  advantage  of  this  duplex  form  of  machine  is 
the  complete  independence  of  the  two  component  parts  in 
their  electrical  relations.  The  motor  part  can  be  designed 
for  any  desired  voltage  or  number  of  alternations.  It  can 
often,  except  in  very  long  transmissions,  take  the  line  voltage 
directly  without  need  for  reducing  transformers,  while  the 
number  of  alternations  can  be  chosen  solely  with  reference  to 
general  conditions  and  without  considering  the  direct  current 
end  of  the  machine  at  all.  This,  as  will  be  seen  when  we  have 
considered  some  other  types  of  current  reorganizers,  is  a  very 
valuable  property,  since  it  gives  the  power  of  obtaining  con- 


276  ELECTRIC  TRANSMISSION  OF  POWER. 

tinuous  current  in  a  thoroughly  practical  way  from  alternating 
currents  of  any  frequency.  Other  reorganizes  can  be  worked 
to  advantage  only  within  a  somewhat  limited  range  of  fre- 
quency. Again,  the  motor-dynamo  can  be  compounded  on 
the  continuous  current  side  without  in  any  way  reacting  upon 
the  alternating  circuit,  and  the  two  circuits  can  be  regulated 
independently  in  any  desired  manner.  All  difficulties  due  to 
lagging  current  can  be  eliminated,  and  the  continuous  current 
side  can  be  kept  at  constant  pressure  irrespective  of  loss  in 
the  main  line  or  any  variations  of  voltage  or  phase  occur- 
ring in  it. 

Finally  the  apparatus  can  as  readily  give  alternating  current 
from  continuous,  as  the  reverse,  and  with  the  same  indepen- 
dence in  each  case. 

The  compensating  disadvantages  are  high  first  cost  and 
rather  large  loss  of  energy  in  the  double  transformation.  As 
to  the  former  count,  it  may  be  said  that  the  advantages  gained 
in  possible  range  of  frequency  and  flexibility  in  the  matter  of 
voltage  go  far  to  offset  the  increase  of  cost.  Often  such 
a  motor-dynamo  is  the  only  possible  way  of  securing  the 
necessary  current.  For  example,  if  one  wished  continuous 
current  for  heavy  motor  service,  such  as  hoists  and  the  like, 
where  the  only  current  available  was  monophase  alternating 
of  125 ~,  or  even  of  6o~  for  that  matter,  the  motor-dynamo 
would  be  the  only  practical  way  of  solving  the  problem. 

As  regards  efficiency  the  motor-dynamo  should  be,  and  is, 
a  little  better  than  motor  and  dynamo  separately,  owing  to 
lessened  friction  of  the  bearings.  Its  efficiency  should  be  as 
great  as  85  per  cent,  at  full  load,  and  might  easily  be  2  or 
3  per  cent,  higher,  in  large  machines.  At  half  load  it  should 
be  say  82  to  85  per  cent.  Practice  too  often  shows  results 
several  per  cent,  below  those  mentioned,  but  this  is  because 
motor-dynamos  have  usually  been  of  very  small  size  and 
sometimes  have  been  made  up  from  any  machines  of  the  right 
speed  that  were  at  hand. 

The  usual  synchronous  motor  may  in  small  motor  genera- 
tors be  replaced  to  advantage  by  an  induction  motor,  which 
is  simpler  than  the  synchronous  form  and  requires  no  brushes. 
Such  a  combination  is  shown  in  Fig.  162.  This  machine  is 
intended  for  supplying  the  current  for  a  large  telegraph  office. 


"•10    .Vt$iU 

t        *        *  '  * 


CURRENT  REORGAN1ZERS.  277 

The  motor  is  a  three-phase  induction  machine  of  2  HP  out- 
put, operated  from  a  no-volt  secondary  at  50^.  The  speed 
is  a  trifle  less  than  1,500  revolutions  per  minute.  With  this 
arrangement  the  attention  required  is  very  trifling,  and 
a  large  number  of  troublesome  batteries  are  displaced.  Of 
late  such  machines  have  assumed  considerable  importance,  and 
many  large  units  have  been  produced.  Plate  VII  shows  in 
Fig.  i  a  500  KW  quarter-phase  set  -running  at  400  r. p.m.  It 
consists  of  an  8  pole  500  KW  railway  generator  coupled  directly 
to  a  20  pole  2200  volt  synchronous  motor,  the  two  machines 
having  a  common  bearing  between  them.  An  interesting 
feature  of  this  set  is  the  exciter  mounted  on  the  same  shaft,  an 
8  KW  multipolar  generator,  so  that  the  whole  outfit  is  self- 
contained.  The  frequency  in  this  case  is66~,  a  periodicity  at 
which  such  motor-generators  have  a  material  advantage  over 
other  apparatus  for  a  like  purpose. 

Fig.  2  is  out  of  the  ordinary  in  that  the  motor  is  of  the 
induction  type,  instead  of  the  ordinary  synchronous  machine. 
Theset  shown  is  of  100  KW  output,  and  comprises  an  ordinary  6 
pole  600  volt  railway  generator  coupled  to  a  12  pole  three-phase 
induction  motor,  running  at  6oor.p.m.,  the  periodicity  being 
6o~.  Induction  motors  have  recently  come  into  considerable 
use  in  this  sort  of  work,  in  spite  of  somewhat  lower  efficiency 
than  the  corresponding  synchronous  motors.  It  is  safe  to  say 
that  the  difference  in  efficiency  is  2  or  3  per  cent.,  and  while 
the  synchronous  motor  may  be  overexcited  so  as  to  improve 
the  power  factor  of  the  system,  the  induction  motor  always 
introduces  lagging  current.  Yet  a  number  of  motor-generators 
with  induction  motors  are  now  being  built  of  capacity  from  500 
to  nearly  1000  KW.  The  real  reason  for  the  use  of  induction 
motors  on  so  large  a  scale  is  the  trouble  which  has  been  experi- 
enced at  many  times  and  places  from  hunting.  These  troubles 
do  not  get  widely  advertised  outside  the  stations  where  they 
occur,  but  it  is  a  fact  that  in  the  use  of  rotary  converters  and 
synchronous  motors  on  a  large  scale  very  serious  and  formi- 
dable developments  of  this  phenomenon  have  occurred,  so  that 
in  spite  of  the  use  of  shields  it  has  under  certain  conditions, 
especially  when  incandescent  lighting  circuits  were  to  be  fed. 
seemed  wise  to  have  recourse  to  induction  motors.  It  is,  how- 
ever, probably  best  to  regard  this  as  a  temporary  expedient, 


278 


ELECTRIC  TRANSMISSION  OF  POWER. 


as  synchronous  motors,  at  least,  can  be  practically  freed  from 
hunting  by  proper  design  and  construction,  and  possess  very 
considerable  advantages.  The  demand  for  machines  of  extreme 
multipolar  construction,  a  demand  based  largely  on  fashion, 
and  the  use  of  laminated  pole  pieces,  are  responsible  for  a 
good  share  of  the  trouble.  Rotary  converters,  as  we  shall  pres 
ently  see,  present  more  serious  problems. 


FIG.  162. 

In  these  large  motor-dynamos  it  is  possible  to  reach  full  load 
efficiencies  in  the  neighborhood  of  90  per  cent.,  and  figures 
fully  up  to  that  point  have  actually  been  obtained.  As  large 
synchronous  motors  can  readily  be  wound  for  10,000  or  12,000 
volts,  under  favorable  conditions  motor-dynamos  can  be  used 
without  reducing  transformers,  which  averts  a  loss  of  2.5  or  3 
per  cent.,  that  would  otherwise  be  incurred. 

From  the  duplex  machines  just  described  it  is  but  a  short 
step  to  the  composite  dynamotor,  so  called,  of  which  the 
armature  is  double  wound.  The  primary  or  high  voltage 
winding  may  of  course  be  either  alternating  or  continuous. 
The  secondary  winding  is  likewise  for  either  current,  and  may 
well  be  fitted  with  both  commutator  and  collecting  rings. 
A  favorite  arrangement  of  the  windings  is  to  place  the 
secondary  coils  in  slots  in  the  armature  core,  apply  a  sheath- 
ing of  insulation,  and  then  to  wind  the  primary  coils  on  the 
smooth  surface  thus  formed.  The  commutators  or  rings. 


CURRENT  REORGAN1ZERS. 


279 


are  placed  one  at  each  end  of  the  armature,  as  in  the  con- 
tinuous current  transformer  shown  in  Fig.  37,  Chap.  III. 

A  typical  dynamotor  of  this  sort  is  shown  in  Fig.  163.  This 
is  specifically  intended  to  derive  a  high  voltage  alternating  cur- 
rent for  testing  purposes  from  a  low  voltage  continuous  cur- 
rent. The  output  is  small,  only  a  fraction  of  a  kilowatt, 
and  the  armature  is  in  the  ordinary  bipolar  field  used  for  small 
motors.  The  motor  or  primary  winding  is  for  no  volts, 
continuous,  and  the  secondary  for  5,000  volts,  alternating.  Of 
course  these  voltages  might  be  anything  desirable,  since  in  so 
small  a  machine  there  are  no  difficulties  in  the  way. 

Another  excellent  specimen  of  the  same  type  is  Fig.  164,  a 
Lahmeyer  "  umformer"  of  about  30  KW  output.  It  is  pri- 
marily a  continuous  current  transformer,  with  675  volts  primary 


FIG.   163. 

and  115  volts  secondary.  It  is  fitted,  however,  as  shown  in 
the  cut,  with  collector  rings  outside  one  of  the  bearings,  from 
which  three-phase  current  at  about  70  volts  can  be  taken. 
There  are  four  field  poles,  and  as  the  normal  speed  is  850 
revolutions  per  minute,  the  three-phase  current  is  at  a  fre- 
quency of  a  little  less  than  30^  per  second. 

This  was  one  of  the  machines  exhibited  at  the    Frankfort 
Exposition  of  1891,  and  fortunately  an  efficiency  test  of  it  is 


280 


ELECTRIC    TRANSMISSION  OF  POWER. 


available,  dealing,  however,  only  with  continuous  currents. 
From  the  nature  of  the  case  the  efficiency  with  a  three-phase 
secondary  would  not  differ  substantially  from  that  found,  so 
that  the  curve,  Fig.  165,  gives  a  closely  approximate  idea  of  the 
general  efficiency  of  such  apparatus  in  the  smaller  sizes.  At 
full  load  the  commercial  efficiency  is  very  nearly  85  per  cent., 
while  at  half  load  it  has  dwindled  to  77  per  cent.  This  is  not 
bad  for  a  small  machine,  and  in  a  unit  of  100  KW  or  more  could 


FIG.  164. 

undoubtedly  be  raised  several  per  cent.  It  should  be  at  least  as 
high  as  can  be  obtained  from  a  duplex  motor  dynamo,  in  fact 
rather  higher,  since  the  bearing  friction  and  core  losses  are 
diminished.  The  composite  machine  is  also  cheaper,  since  but 
one  field  is  used,  and  it  has  a  certain  advantage  in  that  the  arma- 
ture reactions  due  to  the  motor  and  dynamo  windings  tend  to 
oppose  each  other,  and  hence  to  diminish  possible  sparking  and 
disturbance  of  the  field.  It  has  the  same  independence  of  pri- 
mary and  secondary  voltage  as  the  duplex  motor  dynamo. 
On  the  other  hand,  by  reason  of  a  common  field,  the  period- 
icity of  the  currents  in  both  windings  must  be  the  same.  It 


CURRENT  REORGANIZERS. 


281 


must  be  remembered  that  a  continuous  current  armature  has  a 
periodicity  just  as  truly  as  an  alternating  armature.  The  cur- 
rent as  generated  in  each  is  alternating,  but  in  the  former  it  is 
commuted  before  leaving  the  generator.  Now  the  frequency 
of  these  alternations  depends  directly  on  the  number  of  poles 
and  the  revolutions  per  minute,  being  in  fact  the  numerical 
product  of  the  two.  So  if  one  of  these  composite  dynamotors 
be  used  with  the  continuous  current  winding  as  primary,  the 


JO  30 

FIG    165. 

frequency  of  the  alternating  secondary  is  fixed,  since  the 
speed  of  the  machine  cannot  be  changed  without  involving 
both  primary  and  secondary  voltages.  If  the  alternating  cur- 
rent side  be  used  as  the  primary,  the  speed  of  the  machine  is 
fixed  by  the  number  of  alternations,  and  whatever  the  voltage 
of  the  secondary,  the  frequency  must  be  the  same  as  that  of 
the  primary.  Now  it  is  a  fact  well  known  to  dynamo  designers, 
that  continuous  current  dynamos  generating  a  high  frequency 
current  prior  to  its  commutation  are  troublesome  and  costly 
to  build.  Most  continuous  current  dynamos  have  an  intrinsic 
frequency  of  15  to  25 ~  per  second.  To  increase  these  figures 
to  4o~  involves  some  difficulty,  particularly  in  large  machines, 
while  50  or  6o~  are  rather  hard  to  reach,  unless  in  sizes  of  50 
KW  and  below. 

Hence  in  spite  of  the  good  points  of  the  composite  dyna- 
motor,  it  is  of  limited  utility  compared  with  the  duplex  machine 
previously  described,  particularly  since  there  is  a  simpler  way 
of  doing  the  same  work  with  a  higher  efficiency. 


282  ELECTRIC  TRANSMISSION  OF  POWER. 

This  is  found  in  the  so-called  rotary  converter,  (the  name  is 
in  no  wise  descriptive). 

This  machine  is  nothing  more  than  a  continuous  current 
dynamo  fitted  with  collecting  rings  in  addition  to  the  com- 
mutator. These  rings  are  connected  to  appropriate  points  of 
the  armature  winding,  and  supplied  with  alternating  currents 
of  the  same  frequency  which  would  be  generated  by  the  arma- 
ture if  the  machine  were  used  as  a  dynamo.  The  brushes 
being  raised,  the  machine  is  nothing  but  a  synchronous  motor 
running  without  load  at  its  normal  speed.  Now  when  the 
brushes  are  put  down,  the  alternating  current  simply  flows 
through  the  armature  just  as  if  it  were  generated  therein,  is 
commuted  and  passes  out  upon  the  line.  This  commutation 
takes  place  under  just  the  same  general  conditions  as  if  the 
machine  were  used  as  a  generator.  Meanwhile  a  portion  of 
the  current  supplied  is  passing  as  before,  not  through  the 
brushes  but  through  the  winding  to  the  collecting  rings,  keep- 
ing up  the  action  as  a  motor.  Of  the  total  current  then,  a 
small  part  forces  its  way  against  the  E.  M.  F.  set  up  in  the 
windings  by  the  field,  and  supplies  the  motor  function;  a  far 
greater  part,  in  amount  determined  by  the  resistance  and 
inductance  of  the  armature,  flows  as  if  urged  by  this  E.  M.  F., 
to  the  brushes,  and  supplies  the  generator  function  of  the 
machine.  Thus  a  single  armature  winding  serves  to  drive  the 
armature  and  to  furnish  a  large  output  of  commutated  current. 
And  this  current  is  not  simply  rectified,  but  is  of  exactly  the 
same  character  as  if  generated  in  the  armature. 

The  character  of  the  winding  in  a  rotary  converter  is  gen- 
erally precisely  the  same  as  in  a  continuous  current  generator, 
the  only  addition  being  two  or  more  leads  from  symmetrically 
placed  points  in  the  winding  to  the  collecting  rings.  These  leads 
can  be  so  arranged  as  to  form  a  monophase  system  for  the  alter- 
natingcurrent  or,  if  desired,  a  two-  or  three-phase  system.  The 
latter  forms  are  generally  preferred,  since  like  the  correspond- 
ing synchronous  motors  they  can  be  made  self-starting,  while 
the  monophase  machine  has  to  be  brought  to  speed  by  special 
and  by  no  means  simple  methods.  Fig.  166  shows  the  character 
of  the  armature  in  a  simple  bipolar  rotary  converter  (mono- 
phase). Here  the  continuous  current  winding  is  a  Gramme 
ring  in  16  sections.  From  the  brushes  B,B,  continuous  cur- 


CURRENT  REORGANIZERS. 


283 


rent  may  be  applied  or  withdrawn,  while  the  brushes  on  the  col- 
lecting rings  C,C,  perform  the  same  office  for  the  alternating 
current.  Such  a  machine  may  serve  a  variety  of  purposes  as 
follows:  i.  Continuous  current  dynamo.  2.  Alternating  cur- 
rent dynamo.  3.  Continuous  current  motor.  4.  Synchronous 
alternating  motor.  5.  Continuous-alternating  converter.  6. 
Alternating-continuous  converter. 

Diphase  rotary  converters  are  usually  supplied  with  four 
collecting  rings  connected  to  form  two  circuits,  each  one  join- 
ing the  windings  in  two  opposite  quadrants  of  the  armature. 


FIG.  166. 

Triphase  transformers  generally  have  three  collecting  rings, 
with  their  respective  leads  tapped  into  the  windings  120° 
apart.  The  connections  vary  somewhat  for  different  kinds  of 
armature  windings,  but  are  the  same  in  effect  as  those  just 
indicated.  One  of  the  early  practical  machines  of  this  sort 
•exhibited  at  the  Frankfort  Exposition  of  1891  is  shown  in  Fig. 
167.  It  is  of  the  flat  ring  type  usual  to  dynamos  of  Schuckert 
make,  and  is  fitted  with  four  collecting  rings  outside  the  bear- 
ing at  the  commutator  end.  The  rings  were  arranged  for 
either  monophase  or  diphase  connection.  The  rotary  converter 
thus  organized  attracted  great  attention,  and  was  successfully 
operated  in  its  manifold  and  diverse  functions.  It  should 
be  noted  that  if  driven  as  a  dynamo,  such  a  machine  can  furnish 
continuous  and  alternating  current  simultaneously,  a  property 
sometimes  convenient,  and  now  not  infrequently  utilized. 


284 


ELECTRIC  TRANSMISSION  OF  POWER. 


These  rotary  converters  in  the  diphase  and  triphase  forms 
are  playing  a  very  important  part  in  electric  railway  operations 
involving  considerable  distances,  and  a  large  number  of  them 
are  in  highly  successful  use.  A  good  idea  of  the  modern  type 
of  rotary  converter  is  shown  in  Fig.  2,  Plate  VIII.  This  is  one  of 
the  400  KW  machines  installed  in  1894  to  operate  the  electric 
railways  in  the  city  of  Portland,  Ore.  It  is  designed  to  deliver 
continuous  current  at  nearly  600  volts,  and  receives  its  energy 
from  Oregon  City,  about  fourteen  miles  away,  where  is 
installed  a  triphase  transmission  plant.  The  motive  power  is 


Fin.    167. 

derived  from  the  great  falls  of  the  Willamette  PJver.  Current 
is  generated  at  6,000  volts,  with  a  frequency  of  33~  per  second,, 
and  is  given  to  the  rotary  converters  at  about  400  volts, 
from  the  secondaries  of  the  reducing  transformers.  Fig.  i, 
Plate  VIII,  shows  a  250  KW  Westinghouse  diphase  machine, 
adapted  for  use  on  a  6o~  circuit  and  giving  continuous  current 
at  250  volts.  An  interesting  feature  of  this  machine  is  the 
diphase  induction  motor  with  its  armature  on  an  extension  of 
the  main  shaft.  This  serves  to  bring  the  machine  to  speed 
without  calling  for  the  excessive  current  that  would  be  required 
if  the  main  lines  were  closed  upon  the  converter  armature 


*»    •*••••••      •«•  I  •»  •  *I  »  »* 


CURRENT  REORGAN2ZERS.  285 

itself.  The  monophase  form  of  this  very  interesting  apparatus 
has  not  yet  come  into  much  practical  use,  not  through  any  in- 
herent faults,  but  because  most  of  the  power  transmission  has 
so  far  been  accomplished  with  diphase  and  triphase  currents. 

The  efficiency  of  these  machines  is,  as  might  be  expected 
from  their  character,  practically  the  same  as  ordinary  con- 
tinuous current  dynamos  of  the  same  output, or  rather  better 
on  account  of  the  shorter  average  path  for  the  current  in  the 
armature.  In  fact,  so  far  as  general  properties  go,  they  are 
dynamos.  They  furnish  at  present  by  far  the  most  available 
means  of  deriving  continuous  from  alternating  currents,  for 
they  are  simple,  of  great  efficiency,  and  of  about  the  same  price 
as  other  generators  of  the  same  capacity.  In  point  of  fact,  a 
well-designed  polyphase  rotary  converter  has  rather  better 
output  and  efficiency  than  the  corresponding  generator,  since 
for  the  reason  just  noted  the  armature  losses  are  diminished. 
Bearing  this  in  mind,  it  is  apparent  that  increasing  the  number 
of  points  at  which  the  armature  is  tapped  for  the  alternating 
current  supply,  thus  shortening  the  average  path  to  the  brushes, 
will,  other  things  being  equal,  lessen  the  armature  loss.  In 
practice  it  is  found  that  a  three-phase  converter  with 
three  armature  taps  is  considerably  better  than  a  monophase 
converter  with  two;  a  quarter-phase  converter  with  four  is 
somewhat  better  still,  while  a  three-phase  connection  with 
separate  phases  and  six  taps  gives  even  a  higher  output  and 
efficiency.  The  net  result  is  that  while  a  monophase  con- 
verter is  rather  inferior  to  the  corresponding  dynamo  the 
two-  and  three-phase  converters  are  considerably  better  than 
the  corresponding  dynamos.  Quarter-phase  converters  are 
always  connected  for  four  collecting  rings,  and  large  three- 
phase  converters  not  infrequently  have  six,  to  gain  the  advan- 
tage just  mentioned. 

Efficiencies  as  great  as  96  per  cent,  at  full  load  have  been 
obtained  from  large  rotary  converters,  with  93.6  per  cent  at 
half  load.  These  figures  are  from  a  three-phase,  six  collect- 
ing ring  converter  of  nearly  1000  KVV  output. 

As  already  indicated,  there  is  a  strong  tendency  toward  the 
use  of  low  periodicity,  25  ~  to  30~  in  rotary  converters.  This 
is  partially  due  to  the  complication  of  the  commutator  in  high 
frequency  converters,  partly  to  the  current  fashion  for 


286  ELECTRIC    TRANSMISSION  OF  POWER. 

extremely  low  rotative  speeds,  and  partly  to  lack  of  finesse  on 
the  part  of  the  average  designer.  That  converters  for  a  fre- 
quency as  high  as  6o~  are  entirely  feasible  even  in  capacities 
up  to  several  hundred  kilowatts  admits  of  no  discussion,  as 
the  machine  put  in  evidence  in  Plate  VIII,  of  which  a  number 
.are  in  successful  operation,  plainly  shows.  It  is  undoubtedly 
easier  to  build  them  for  somewlrat  lower  periodicities,  but 
there  seems  very  little  reason  for  going  so  low  as  is  the  cur- 
rent custom,  and  it  tends  needlessly  to  multiply  special  types 
of  apparatus. 

And  yet  the  simplicity  of  the  rotary  converter  is  attained 
at  the  cost  of  certain  practical  inconveniences  that  cannot 
lightly  be  passed  by.  Their  source  is  the  employment  of  a 
single  field  and  armature  winding  for  all  the  purposes  of  the 
apparatus.  The  results  are,  first,  complete  interdependence 
of  the  alternating  and  continuous  voltages,  and,  second,  con- 
sequent difficulties  of  regulation  that  are  occasionally  very 
troublesome. 

The  immediate  result  of  a  single  winding  is  that  there  is  an 
approximately  fixed  ratio  between  the  alternating  and  the  con- 
tinuous voltage.  The  former  is  always  the  less,  and,  while 
varied  by  changes  in  the  number  of  phases  determined  by  the 
connections,  is  approximately  the  alternating  voltage  that 
would  be  yielded  by  the  machine  driven  as  a  generator.  This 
is,  for  monophase  or  diphase  connections,  about  seven-tenths 
of  the  continuous  current  voltage,  and  for  three-phase  connec- 
tions about  six-tenths.  The  proportions  would  approximate  to 

JL^  an(j    -Y^L.  respectively,  if  the  alternating  E.  M.  Fs.  were 

V/2  2V/2 

sine  waves,  which  they  never  are  when  derived  from  an  ordi- 
nary continuous  current  armature.  In  service  the  real  pro- 
portions may,  and  generally  do,  vary  by  several  per  cent., 
according  to  the  excitation.  In  a  particular  two-phase  case 
the  actual  ratio  was  .68,  and  in  a  three-phase  case  .65.  If, 
therefore,  a  rotary  converter  be  used  for  supplying  continuous 
current,  the  applied  alternating  current  must  be  of  lower  pres- 
sure than  the  derived  continuous,  in  about  the  proportion 
above  noted.  This  compels  the  use  of  reducing  transformers 
in  every  case  of  power  transmission  involving  this  apparatus. 


CURRENT  REORGANIZERS.  287 

Further,  any  cause  that  affects  the  alternating  pressure  affects 
the  continuous  as  well.  Line  loss,  inductance,  resonance  ef- 
fects, as  well  as  changes  at  the  generators,  all  influence  the  volt- 
age at  the  continuous  current  end  of  the  rotary  transformer. 
Nor  can  this  voltage  be  freely  altered  by  changing  the  field 
strength  of  the  rotary  transformer,  since  as  we  have  already 
seen  this  may  profoundly  change  the  inductance  of  the  alter- 
nating circuit,  which  is  for  many  reasons  undesirable.  There- 
fore compound  winding,  while  perfectly  practicable,  may  cause 
trouble.  The  best  results  are  obtained  by  carefully  adjusting 
the  generator,  line,  and  rotary  transformer  to  work  together. 
Otherwise  there  is  very  likely  to  be  trouble  in  regulation. 

For  these  reasons  in  cases  where  close  regulation  is  neces- 
sary, as  for  incandescent  lighting,  preference  has  frequently 
been  given  to  the  motor  generator  with  double  field  and  arma- 
ture, as  in  the  large  Budapest  plant  installed  by  Schuckert  & 
Co.,  who  were  among  the  pioneers  ir>  developing  the  rotary 
transformer.  In  this  case  the  transmission  is  at  2,000  volts 
diphase,  at  which  pressure  current  is  delivered  to  the  motor 
end  of  the  motor  generators  placed  in  sub  stations  at  conven- 
ient points.  In  such  a  plant  the  increased  cost  of  the  duplex 
machines  is  not  so  great  as  might  be  supposed,  for  reducing 
transformers  are  needless,  and  the  output  of  both  generators 
.and  motors  can  be  forced  to  the  utmost  limit  of  efficient  oper- 
ation, without  fear  of  injuring  the  regulation,  which  is  reduced 
to  the  easy  problem  of  accurately  compounding  a  continuous 
current  generator.  The  net  efficiency  of  the  Budapest  trans- 
formation is  said  to  be  85  per  cent.  Some  recent  experiments 
•on  the  relative  efficiency  and  cost  of  motor  generators  and 
rotary  converters  are  as  follows:  The  se$s  compared  were  of 
200  KW  capacity  for  changing  tri-phase  current  from  the 
Niagara  circuits  at  n,ooo  volts,  25 ~  into  continuous  current 
at  120  to  150  volts.  The  efficiencies  given  are  net,  including 
the  necessary  provisions  for  obtaining  a  variation  of  25  per 
cent,  in  the  finally  resulting  voltage: 

MOTOR-  TRANSFORMERS  DIFFERENCE. 

GENERATOR.  AND     ROTARIES. 

Full  load  87.40  89.87  2.47$ 

%  load  85.54  88.70  3-i6£ 

*/2  load  81.42  84.90  3.48^ 


288  ELECTRIC    TRANSMISSION  OF  POWER. 

The  extra  apparatus  required  with  the  rotaries  brought  the 
two  methods  to  substantially  the  same  cost,  but  for  lighting 
work  the  motor  generators  gave  the  better  results. 

From  the  foregoing  it  is  sufficiently  evident  that  every  case  of 
current  reorganization  cannot  be  successfully  met  by  the  same 
apparatus.  For  arc  lighting  at  low  frequency  the  rotating 
commutator  seems  to  be  fairly  well  suited,  and  for  that  par- 
ticular purpose  it  is  somewhat  cheaper  and  more  efficient  than 
any  of  its  rivals.  Next  in  point  of  efficiency  and  cheapness 
comes  the  rotary  converter,  infinitely  better  for  heavy  work 
than  any  commutating  device,  and  finding  already  extensive 
application  to  electric  railway  work.  Finally,  for  work  requir- 
ing very  close  regulation,  the  motor  generator  is  specially  well 
suited,  closer  to  the  rotary  transformer  in  cost  and  efficiency 
than  would  be  supposed  offhand,  and  unique  in  the  complete 
independence  of  its  working  circuits. 

Practice  in  this  line  of  operations  has  not  yet  settled  into 
fixed  directions,  and  is  not  likely  so  to  do  just  at  present. 
Each  plant  must  therefore  be  considered  by  itself  and  treated 
symptomatically. 

American  usage  is  at  present  tending  strongly  toward  the 
rotary  converter,  on  account  of  its  ready  adaptation  to  railway 
service,  but,  in  view  of  the  work  that  has  been  done  on  alternat- 
ing motors  for  such  service,  it  is  an  open  question  how  far 
current  reorganization  will  be  generally  necessary  in  the  future, 
although  just  now  it  is  of  very  great  practical  importance. 

As  the  price  of  copper  rises  the  use  of  current  reorganizers 
becomes  more  and  more  important  in  railway  work,  and  for 
this  particular  use  the  rotary  converter  is  generally  chosen. 

There  should  be  mentioned  here  some  curious  devices  for 
obtaining  rectified  alternating  currents  based  upon  the  phe- 
nomena of  electrolytic  polarization. 

Obviously  if  one  could  find  a  conductor  which  would  let  pass 
currents  in  one  direction,  and  block  those  in  the  other,  the 
result  of  putting  it  in  an  alternating  circuit  would  be  that  all 
the  current  impulses  in  one  direction  would  be  suppressed,  so 
that  the  resulting  current  would  be  a  series  of  separated  half- 
waves  of  the  same  polarity.  It  would  be  as  if  in  Fig.  155  all 
the  half-waves  above  the  base  line  were  erased.  Now  such  a 
conductor  is  actually  obtainable  in  certain  electrolytic  cells  in 


CURRENT  REORGANIZERS.  289 

which  a  counter  electromotive  force  or  severe  polarization 
resistance  impedes  current  flowing  in  a  particular  direction. 
Under  favorable  circumstances  the  selective  action  is  quite 
complete,  so  that  the  alternating  current  becomes  unidirec- 
tional. Fig.  168  shows  the  current  curve  for  a  complete 
cycle  as  modified  by  electrolytic  rectification.  The  positive 
half  of  the  wave  is  practically  wiped  out  of  existence.  The 


FIG.  168. 

efficiency  of  these  devices  as  regards  the  energy  rectified  is 
quite  low,  and  most  of  the  apparatus  constructed  has  been 
upon  a  very  small  scale,  but  there  are  certain  purposes,  like 
energizing  induction  coils,  for  which  it  might  occasionally  be  of 
service.  It  is  given  place  here  more  on  account  of  its  general 
interest  than  for  any  practical  value.  It  works  best,  like  other 
rectifying  devices,  at  low  frequencies. 


CHAPTER  VIII. 

ENGINES   AND    BOILERS. 

MECHANISMS  that  constitute  the  link  between  natural  sources 
of  energy  and  mechanical  power  are  called  prime  movers.  So 
far  as  the  electrical  transmission  of  energy  is  concerned,  but 
two  classes  of  prime  movers,  steam  engines  and  water-wheels, 
have  to  be  seriously  considered.  All  others  sink  into  insignif- 
icance or  are  limited  to  special  and  rarely-occurring  cages. 
When  power  is  transmitted  electrically  over  considerable  dis- 
tances the  prime  mover  is  usually  a  water-wheel,  since,  as  yet, 
the  transmission  of  power  from  coal  fields  has  been  hardly 
more  than  begun,  although  when  long  electrical  lines  be- 
come somewhat  more  familiar,  coal  may  become  a  frequent 
source  of  energy.  Where  the  distribution  of  power  from  a 
central  point  is  to  be  accomplished,  the  prime  mover  is  fre- 
quently a  steam  engine. 

The  general  principle  of  the  steam  engine  may  be  fairly 
supposed  to  be  somewhat  familiar  to  the  reader,  but  the  con- 
ditions of  economy  are  not  always  so  clearly  understood.  The 
source  of  power  in  an  engine  is  the  pressure  of  the  steam, 
which  must  be  utilized  as  fully  as  possible  to  get  anything  like 
efficient  working.  Since  the  pressure  is  in  direct  proportion 
to  the  temperature  in  any  gas,  the  proportion  of  the  total  pres- 
sure which  can  be  used  depends  on  the  original  temperature 
at  which  its  use  is  begun,  and  the  temperature  at  which  one 
ceases  to  use  it  and  rejects  it  together  with  all  the  energy  it 
then  possesses.  These  temperatures  are  not  to  be  reckoned 
from  the  ordinary  zero  of  a  thermometer,  but  from  the  so- 
called  absolute  zero.  This  is  that  point  from  which,  if  the 
temperature  of  a  gas  be  reckoned,  its  pressure  will  be  directly 
proportional  to  the  temperature.  It  is  461°  below  zero, 
Fahrenheit,  that  is,  493°  below  the  melting  point  of  ice.  It  is 
determined  by  the  consideration  that  any  gas  at  this  melting 
point  loses  ^-J-^-  of  its  pressure  for  a  change  in  temperature  of 


ENGINES  AND  BOILERS.  291 

one  degree,  hence  if  it  could  be  cooled  down  493°,  would  lose 
its  pressure  and  would  have  given  up  all  of  its  energy.  Count- 
ing from  this  absolute  zero  then,  one  can  utilize  that  part  of 
the  whole  energy  of  a  gas  which  lies  between  the  temperature 
at  which  the  gas  begins  to  work  and  that  at  which  it  ceases  to 
do  work.  In  other  words  the  efficiency  of  any  engine  operated 
by  gaseous  pressure  is: 


In  which  71,  is  the  absolute  temperature  of  the  gas  when 
it  begins  to  do  work  in  the  engine,  and  7*a  the  absolute 
temperature  at  which  its  work  ends.  In  practice  Tl  is  the 
temperature  of  the  steam  when  it  enters  the  cylinder,  and  T9 
the  temperature  of  exhaust  or  condensation.  Steam  permits 
the  use  of  but  a  limited  range  of  temperature  on  account 
of  the  temperature  at  which  it  liquefies,  and  bothers  us  by 
condensing  as  it  expands,  even  in  the  cylinder.  It  must  be 
remembered  that  while  we  are  limited  by  our  possible  range  of 
temperature  to  a  low  tocal  efficiency  in  any  heat  engine,  of  the 
energy  that  can  possibly  be  obtained  within  this  limitation, 
a  very  good  'proportion  is  recovered  in  the  best  modern 
engines  —  from  one-half  to  two-thirds.  The  remainder  is  lost 
in  various  ways,  largely  through  radiation  of  heat  and  cylinder 
condensation.  Besides  these  thermal  losses  a  portion  of  the 
energy  utilized  is  wasted  in  friction  of  the  mechanism. 

From  these  considerations  we  may  derive  the  following 
general  principles  of  engine  efficiency: 

I.  The  steam  should  be  admitted  at  the   highest  pressure 
feasible  and  exhausted  at  the  lowest  pressure  possible. 

This  indicates  that  high  boiler  pressure  should  be  used,  and 
that  it  is  better  to  condense  the  steam  than  to  expel  it  into  the 
air,  as  by  condensing  most  of  the  atmospheric  pressure  can 
be  added  to  the  working  range  of  pressure  in  the  engine.  In 
the  next  place  it  is  evident  that  the  steam  should  be  sent  into 
the  engine  at  full  boiler  pressure,  and  finally  condensed  after 
expanding  and  yielding  up  its  pressure  as  completely  as 
possible. 

II.  Waste  of  heat  in  the  engine  should  be  stopped  as  far 
as  possible.     This  means  checking  losses  from  the  cylinder 
by  radiation  and    conduction,  and  internal  loss  from  cylinder 


292  ELECTRIC   TRANSMISSION   OF  POWER. 

condensation.  The  first  principle  laid  down  has  for  its  ob- 
ject the  increase  of  the  possible  efficiency,  while  this  second 
principle  bears  on  the  securing  of  as  large  a  proportion  as 
possible  of  this  possible  efficiency.  It  requires  the  preven- 
tion of  escape  of  heat  externally  by  protecting  the  cylinder, 
and  incidentally  shows  the  advantage  of  high  pressure  and  high 
piston  speed  in  securing  as  much  work  as  possible  without  in- 
creasing the  size  of  the  working  parts,  and  hence  their  chance 
for  radiation.  On  the  other  hand  it  indicates  the  danger  of 
working  with  too  great  a  range  of  temperature  in  the  cylinder, 
thus  producing  cylinder  condensation. 

III.  The  work  of  the  engine  should  be  the  maximum  practi- 
cable for  its  dimensions  and  use.  .  This  secures  high  mechan- 
ical efficiency  as  the  previous  principles  secure  high  thermal 
efficiency.  To  fulfill  this  condition  high  steam  pressure  and 
high  piston  speed  are  necessary,  and  the  latter  usually  means 
also  rather  high  rotative  speed.  The  importance,  too,  of  fine 
workmanship  in  the  moving  parts  is  evident. 

It  will  be  realized  that  some  of  the  conditions  just  pointed 
out  are  mutually  incompatible  to  a  certain  extent.  Every- 
thing points,  however,  to  the  great  desirability  of  a  condens- 
ing engine,  worked  with  a  high  initial  steam  pressure  and 
great  piston  speed.  The  tendency  of  the  best  modern  prac- 
tice is  all  in  this  direction,  and  the  efficiency  of  engines  is  con- 
stantly improving.  The  greatest  advances  of  the  past  decade 
or  two  have  been  in  the  introduction  of  compound  engines. 
The  principle  here  involved  is  the  lessening  of  thermal 
losses  in  the  cylinder  by  avoiding  extremes  of  temperature 
between  the  initial  and  the  final  temperature  of  the  steam  ex- 
panded into  it.  Compound  engines  simply  divide  the  expan- 
sion of  the  steam  between  two  or  more  cylinders,  so  that  the 
temperature  range  in  each  is  limited,  without  limiting  the 
total  amount  of  expansion. 

Following  the  same  line  of  improvement,  triple  and  quad- 
ruple expansion  engines  are  becoming  rather  common,  although 
the  value  of  the  last  mentioned  is  somewhat  problematical  at 
present. 

For  practical  purposes  steam  engines  may  be  classified  in 
terms  of  their  properties,  somewhat  as  follows: 

First,  there  is  the  broad  distinction  between  condensing  and 


ENGINES  AND  BOILERS.  293 

non-condensing  engines.  The  former  condense  the  exhausted 
steam  and  gain  thereby  a  large  proportion  of  the  atmospheric 
pressure  against  which  the  latter  class  is  obliged  to  do  work  in 
exhausting  the  steam.  Where  economy  of  operation  is  se- 
riously considered,  the  non-condensing  engine  has  no  place,  if 
water  for  condensation  is  obtainable. 

Each  of  these  classes  falls  naturally  into  sub-classes,  depend- 
ing on  the  number  of  steps  into  which  the  expansion  is 
divided — simple,  compound,  triple  expansion,  etc.  Of  these 
the  first  may  now  and  then  be  desirable,  where  the  size  is  small 
.and  coal  very  cheap,  but  for  the  general  distribution  of  energy 
the  last  two  are  more  generally  useful.  Furthermore  each  of 
the  sub-classes  mentioned  may  be  divided  into  two  genera, 
depending  on  the  nature  of  the  valve  motions  that  control  the 
admission  and  rejection  of  the  steam.  To  follow  out  the  first 
principle  of  economy  laid  down,  the  steam  must  be  admitted 
at  a  uniform  pressure  as  near  that  of  the  boiler  as  possible,  the 
admission  should  be  stopped  short  after  entrance  of  enough 
.steam  for  the  work  of  the  stroke,  the  steam  allowed  to  expand 
the  required  amount,  and  then  rejected  completely  at  the  lowest 
possible  pressure.  The  acrmission  valves  should  therefore 
•open  wide  and  very  rapidly,  let  in  the  steam  for  such  part  of 
the  stroke  as  is  necessary,  and  then  as  promptly  close.  The 
exhaust  valves  should  open  quickly  and  wide  when  the  expansion 
is  complete,  and  stay  open  until  nearly  the  end  of  the  stroke, 
closing  just  soon  enough  to  cushion  the  piston  at  the  end  of  its 
stroke.  In  proportion  to  the  completeness  with  which  these 
conditions  are  met  the  use  of  the  steam  will  be  economical  or 
wasteful.  The  two  genera  of  engines  referred  to  are  those 
in  which  the  motions  of  the  admission  and  exhaust  valves  are 
independent  of  each  other  or  dependent.  Fig.  169  shows  in 
section  the  cylinder  and  valves  of  an  independent  valve  engine, 
Corliss  type.  The  arrows  show  the  flow  of  the  steam.  The 
admission  valve  on  the  head  end  of  the  cylinder  has  just  been 
opened,  as  also  has  the  exhaust  valve  on  the  crank  end. 

The  essential  point  of  the  mechanism  is  that  the  admission 
valves  open  and  close  at  whatever  time  is  determined  by  the 
action  of  the  governor  without  in  the  least  affecting  the  work- 
ing of  the  exhaust  valves.  In  the  Corliss  valve  gear  the 
steam  vnlves  are  closed  by  gravity,  or  by  a  vacuum  pot,  and 


294 


ELECTRIC   TRANSMISSION  OF  POWER. 


are  opened  by  catches  moved  by  an  eccentric  rod,  and  released 
at  a  point  determined  by  the  governor,  which  thus  varies  the 
point  of  cut-off  according  to  the  load.  Ordinarily  the  admis- 
sion of  steam  is  thus  cut  off  in  a  simple  engine  at  full  load 
after  the  piston  has  traversed  from  one-fifth  to  one-quarter  of 
its  stroke,  according  to  the  pressure  of  the  steam.  If  the  cut- 
off is  too  late  in  the  stroke,  there  is  not  sufficient  expansion  of 
the  steam  ;  if  too  early  the  steam  is  partially  condensed  by  too 
great  expansion.  For  every  initial  pressure  of  steam  there  is  a 
particular  degree  of  expansion  which  gives  the  best  results  in  a 
given  engine. 

\ 


FIG.  169. 

Fig.  170  shows  the  valve  motion  of  one  of  the  best  of  the- 
dependent-valve  genus.  Steam  is  just  being  admitted  at  the 
head  end  both  around  the  shoulder  of  the  hollow  piston  valve 
and  through  the  ports  at  the  other  end  of  the  valve  via  the 
interior  space.  At  the  crank  end  the  exhaust  port  has  just 
been  fully  opened.  It  will  be  seen  that  any  change  in  the 
conditions  of  admission  also  involves  a  change  in  the  condi- 
tions of'  exhaust,  and  although  some  variation  may  take  place 
in  the  latter  without  serious  result  on  the  economy,  simplicity 
in  the  valve  gear  has  been  gained  at  a  certain  sacrifice  of 
efficiency  in  using  the  steam.  Both  independent  and  depend- 
ent valve  engines  have  many  species  differing  widely  in 
mechanism,  but  retaining  the  same  fundamental  difference. 
Of  the  two  genera,  the  independent-valve  engine  has  the 
material  advantage  in  efficiency,  and  under  similar  conditions 


ENGINES  AND  BOILERS. 


295 


of  pressure,  capacity,  and  piston  speed  consumes  from  10  to  20 
per  cent,  less  steam  for  the  same  effective  power.  It  there- 
fore is  generally  employed,  in  spite  of  somewhat  greater  first 
cost,  for  all  large  work,  often  in  the  compound  or  triple- 
expansion  form.  Except  in  small  powers,  or  for  exceptionally 
high  speed,  the  dependent-valve  engine  has  few  advantages, 
and  in  the  generation  of  power  on  a  large  scale,  such  as  for 
the  most  part  concerns  us  in  electrical  transmission  work,  it 
hardly  has  an  important  place. 


FIG.  170. 

It  must  not  be  supposed  that  between  the  various  sorts  of 
engines  mentioned  there  are  hard  and  fast  lines.  In  the 
economical  use  of  steam  a  very  large  non-condensing  engine 
may  surpass  a  smaller  condensing  one,  or  a  fast  running 
dependent-valve  engine,  a  very  slow  running  one  with  inde- 
pendent valves.  Broadly,  however,  we  may  lay  down  the 
following  propositions  concerning  engines  of  similar  capacity: 

I.  Condensing  engines  will  always  furnish  power  more 
economically  than  non-condensing  ones.  This  is  particularly 


296  ELECTRIC   TRANSMISSION  OF  POWER. 

true  at  less  than  full  load,  since  the  loss  of  the  atmospheric 
pressure  may  be  taken  as  a  constant  source  of  inefficiency, 
which,  like  mechanical  friction,  is  very  serious  at  low  loads. 
For  example,  a  triple-expansion  engine  working  at  one-quarter 
load  in  indicated  HP,  will  be  likely  to  have  its  consumption  of 
steam  per  IHP,  increased  from  15  to  25  per  cent,  above  the  con- 
sumption per  IHP  at  full  load;  while  worked  non-condensing, 
the  increase  would  be  from  50  to  100  per  cent.  Hence  for 
electrical  working  where  light  loads  are  frequent,  condensing 
engines  are  an  enormous  advantage.  With  simple  or  compound 
engines  the  same  general  rule  holds  good  as  for  triple-expan- 
sion engines,  with  the  additional  point  that  light  loads  affect 
their  economy  even  more,  when  worked  non-condensing.  It 
must  be  borne  in  mind  that  if  any  engine  is  to  do  its  best  under 
varying  loads,  its  valve  gear  and  working  pressure  must  be 
arranged  with  this  in  mind,  else  the  advantage  of  high  expan- 
sion and  condensing  may  be  thrown  away.  It  is  frequently 
said  that  triple-expansion  engines  do  not  give  good  results  in 
electric  railway  work.  When  this  is  the  case  there  has  been 
improper  adjustment  of  engine  to  load. 

II.  Among  engines   having  the  same  class  of  valve  gear, 
compound  engines  give  better  economy  than  simple  ones,  and 
triple-expansion   better  than    compound.     This   is   true  irre- 
spective of  the  nature  of  the  load,  supposing  each  engine  to  be 
suitably  adjusted  to  the  work  it  has  to  do.     In  rare  cases,  owing 
to  exceedingly  cheap  fuel  and  short  working  hours,  it  may  hap- 
pen  that  the  advantage   of  a  triple-expansion  engine  over  a 
compound  in   economy  of  coal   may  be   more  than  offset  by 
increased  interest  on  investment,  but  at  the  present  cost  of 
engines  and   boilers,  this  could   not  well  occur  unless  in  the 
case  of   burning   culm  or   poor  coal   obtained   at   a    nominal 
price. 

III.  As  regards  speed  of  engines,  there  is  always  advantage 
in  high  piston  speed  both  as  respects  first  cost  and  mechanical 
efficiency.     So  far  as  the  economical  use  of  steam  goes,  speed 
makes  little  difference  save  as  it  sometimes  involves  a  change 
in  the  valve  gear.      Most  high-speed  engines  have  valve  gear 
of  the   dependent  sort,    which  puts  them  at  a  disadvantage 
except  in  so  far  as  lessened  cylinder  condensation  and  friction 
may  offset  the  losses  due  to  less  efficient  distribution  of  the 


ENGINES  AND   BOILERS.  297 

steam.  But  the  best  dependent-valve  engine  is  uniformly  less 
economical  than  the  best  independent-valve  engine  of  the 
same  class  and  sub-class.  Even  the  lessened  friction  of  the 
small  high-speed  pistons  does  not  offset  this  difference  in 
intrinsic  economy. 

As  regards  actual  economy  in  the  steam  consumption,  the 
size  of  engine  has  a  powerful  though  somewhat  indeterminate 
influence.  Even  at  full  load,  simple  non-condensing  dependent- 
valve  engines  of  moderate  size  require  from  30  to  40  Ibs. 
of  steam  per  indicated  horse-power-hour.  Only  in  very  large 
engines,  such  as  locomotives,  and  specially  fast  running  engines 
such  as  the  Willans,  does  the  steam  consumption  of  these 
dependent-valve  engines  fall  below  30  Ibs.,  and  not  very  often 
even  in  these  cases.  Worked  condensing  the  same  machines 
use  from  20  Ibs.  in  exceedingly  favorable  cases,  to  25  or  30  Ibs. 
more  commonly. 

Independent  valve  engines,  simple  and  non-condensing,  will 
give  the  indicated  HPH  on  25  to  30  Ibs.  of  steam,  occasion- 
ally on  as  little  as  22  to  23  Ibs.  With  the  advantage  of  con- 
densation these  figures  may  be  reduced  to  say  18  to  25  Ibs., 
the  former  figure  being  somewhat  exceptional  and  probably 
very  rarely  attained  in  practice. 

Passing  now  to  compound  non-condensing  engines,  the  effect 
of  compounding  on  efficiency  is  about  the  same  as  that  of  con- 
densing. Ordinary  dependent-valve  engines  of  compound 
construction  require  from  20  to  28  or  30  Ibs.  of  steam  per 
IHP  hour.  The  former  result  is  very  exceptional,  and  seldom 
or  never  reached  in  practice,  while  the  last-mentioned  would 
be  considered  rather  high.  Independent-valve  compound 
engines  are  so  seldom  worked  non-condensing,  that  the  data  of 
their  performance  are  rather  meagre;  18  to  25  Ibs.  of  steam 
is  about  the  usual  amount,  however. 

WThen  condensation  is  employed,  on  the  other  hand,  the 
dependent-valve  engines  are  in  rather  infrequent  use.  When 
the  need  for  economy  is  so  felt  as  to  lead  to  the  use  of  com- 
pound engines,  it  also  leads  to  the  use  of  economical 
valve  gear.  The  steam  consumption  of  dependent-valve  com- 
pound-condensing engines  is  quite  well  known,  however,  and  is 
usually  from  16  to  24  Ibs.  per  IHP  hour.  The  first  mentioned 
figure  is  rarely  reached,  and  only  in  special  types  of  engine. 


298  ELECTRIC   TRANSMISSION  OF  POWER. 

Plenty  of  tests  on  compound  condensing  engines  with  inde- 
pendent valves  are  available;  14  to  20  Ibs.  of  steam  covers 
the  majority  of  results.  Occasional  tests  run  as  low  as  13  and 
as  high  as  22  Ibs. 

.  It  is  noticeable  that  in  compound  engines  the  difference 
between  dependent  and  independent  valve  gear  is  less  than 
with  simple  engines.  This  is  due  to  a  variety  of  causes.  The 
larger  range  of  expansion  used  in  compound  engines  tends  to 
lessen  the  deleterious  effects  of  moderate  variations  in  the 
distribution  of  the  steam,  and  besides,  the  valve  gear  of  com- 
pound engines  is  not  infrequently  composite,  the  high-pres- 
sure cylinder  having  independent  valves  and  the  low-pressure 
cylinder  dependent  ones. 

The  same  arrangement  is  often  used  in  triple-expansion 
engines,  so  that,  in  conjunction  with  the  condition  before 
mentioned,  it  is  usually  true  that  the  economy  of  dependent- 
valve  triple-expansion  engines  is  much  nearer  that  of  indepen- 
dent-valve ones  than  would  be  at  first  supposed.  Without 
condensing  a  dependent-valve  triple-expansion  engine  may  be 
expected  to  require  from  19  to  27  Ibs.  of  steam  per  IHP 
hour.  With  condensation  such  engines  perform  much  better, 
the  steam  consumption  being  reduced  to  14  to  20  Ibs. 

Nearly  all  triple-expansion  engines,  however,  are  built  with 
independent  valves  at  least  in  part,  the  intention  being  to 
secure  the  most  economical  performance  possible.  Under 
favorable  conditions  their  steam  consumption  runs  as  low  as 
12  Ibs.  per  IHP  hour,  and  seldom  rises  above  18  Ibs.  In  a 
few  exceptional  cases  the  record  has  been  reduced  slightly 
below  i2  Ibs.,  but  such  results  cannot  fairly  be  expected. 
Anything  under  13  Ibs.  of  steam  per  HP  is  exceedingly 
good  practice  for  running  conditions. 

All  the  figures  given  refer  in  the  main  to  good-sized  engines 
of  at  least  200  HP  and  over,  operated  at  full  load  and  at 
favorabje  ratios  of  expansion.  It  must  be  clearly  understood 
that  there  is  for  each  steam  pressure  a  particular  ratio  of 
expansion  which  will  give  the  most  economical  result — less  ex- 
pansion than  this  rejects  the  steam  at  too  high  a  temperature; 
more,  causes  loss  by  condensation,  etc.  Compound  and  triple- 
expansion  engines  permit  greater  expansion  of  the  steam  with- 
out loss  of  economy,  hence  allow  higher  steam  pressure  and  a 


ENGINES  AND  TOILERS. 


299 


greater  temperature  range — hence  higher  thermal  efficiency. 
Good  practice  indicates  that  for  simple  engines  the  boiler 
pressure  should  be  not  less  than  90  to  100  Ibs.  per  square 
inch,  for  compound  engines  not  less  than  no  to  120,  and  for 
triple-expansion  engines  not  less  than  140  to  150,  and  thence 
up  to  175  or  200  Ibs. 

We  may  gather  the  facts  regarding  steam  consumption  into 
tabular  form  somewhat  as  follows: 


KIND  OF  ENGINE. 

STEAM  PER  I  HP. 
GENERAL  RANGE. 

STEAM  PER  I  HP. 
WORKING  AVERAGE. 

Simple  non-con    dep    v 

•30-40 

33 

25-30 

28 

"        con    dep.  v  

20-30 

25 

44           44    indep   v 

18-25 

21 

Compound,  non-con,  dep.  v.  .  .  . 
"                 "         indep.  v... 
"            con    dep   v  . 

20-28 
18-25 
16—24 

24 
22 
2O 

"               "     indep.  v  

14-20 

17 

Triple   con.  dep.  v  

14-20 

17 

*•          "      indep    v 

12-18 

IS 

"large  4l         "        

12-14 

13 

The  engines  considered  are  supposed  to  be  of  good  size — 
say  200  to  500  HP,  and  to  be  worked  steadily  at  or  near 
full  load.  The  figures  given  as  working  average  are  such  as 
may  be  safely  counted  on  with  good  engines,  kept  in  the  best 
working  condition,  and  operated  at  at  least  the  boiler  pres- 
sures indicated.  The  steam  is  supposed  to  be  practically  dry 
and  the  piping  so  protected  as  to  lose  little  by  condensation. 
These  results  are  such  as  may  regularly  be  obtained  in  prac- 
tice, and  indeed  it  is  not  uncommon  to  find  them  excelled. 
Compound-condensing  engines  of  large  size  not  infrequently 
work  down  to  15  pounds  of  steam,  and  triple-expansion  cor 
densing  engines  down  to  14  pounds,  which  result  will  be  guar- 
anteed by  most  responsible  builders. 

Unfortunately  engines  employed  for  electrical  work  are  com- 
paratively seldom  kept  at  uniform  and  full  load.  Furthermore, 
they  are  subject  to  all  sorts  of  variations  of  load.  In  electric 
railway  service  there  are  sudden  changes  from  light  loads  to 
very  heavy  ones,  while  in  electric  lighting  there  is  generally  a 
gradual  increase  to  the  maximum  load,  which  continues  an 


300  ELECTRIC    TRANSMISSION  OF  POWER. 

hour  or  two,  followed  by  a  rather  gradual  decrease.  These 
variations  affect  the  economy  of  the  engines  unfavorably — at 
certain  loads  there  is  not  enough  expansion,  at  others  decidedly 
too  much.  The  variations  in  economy  are  largely  controlled 
by  the  proportioning  of  the  engine  to  its  work.  To  say  that 
an  engine  is  of  500  HP  means  little  unless  the  statement  be 
coupled  with  a  definite  explanation  of  the  circumstances.  If 
that  output  is  obtained  by  admitting  steam  for  half  the  stroke, 
the  engine  will  work  at  500  HP  very  uneconomically,  sup- 
posing a  simple  engine  to  be  under  consideration.  Its  point 
of  maximum  economy  may  be  perhaps  300  HP.  On  the  other 
hand  500  HP  may  be  given  when  cutting  off  the  steam  at  one- 
fifth  stroke.  In  this  case  the  engine  will  be  working  near  its 
point  of  maximum  economy,  and  at  300  HP  will  require  much 
more  steam  per  IHP.  It  could  give  probably  600  to  700  HP 
at  a  longer  cut-off,  and  is  really  a  much  more  powerful  engine 
than  the  first.  For  uniformity  it  is  better  to  rate  an  engine  at 
the  HP  of  maximum  economy,  whatever  the  real  load  may  be. 
The  relation  of  load  to  economy  is  well  shown  in  the  curves  of 
Fig.  171. 

Curves  i,  2,  4,  and  5,  are  of  engines  so  rated  as  to  have 
their  maximum  economy  near  full  load.  Curve  3,  on  the 
other  hand,  is  from  an  engine  intended  to  give  its  highest 
economy  at  about  three-quarters  load.  For  very  variable 
output  this,  is  the  preferable  arrangement,  while  for  large 
central  station  work,  when  the  number  of  units  is  large  enough 
to  permit  loading  fully  all  that  are  running  at  any  one  time, 
it  is  better  to  have  each  unit  give  its  very  best  economy  near 
full  load  and  to  vary  the  number  of  units  according  to  the  re- 
quirements of  total  load. 

For  electric  railway  service  under  ordinary  conditions,  it  is 
best  to  employ  an  engine  which  at  full  load  is  worked  to 
a  high  capacity,  and  hence  somewhat  uneconomically,  while  at 
lesser  loads,  which  more  nearly  correspond  with  the  average 
conditions,  its  economy  will  be  at  a  maximum.  For  electric 
lighting  service  it  is  preferable  to  have  the  point  of  maximum 
economy  fall  more  nearly  at  full  load.  For  power  service, 
which  is  on  the  one  hand  more  uniform  than  railway  service,  and 
less  uniform  than  electric  lighting  work,  it  is  probably  best  to 
employ  an  engine  having  characteristics  between  those  just 


ENGINES  AND   BOILERS. 


301 


mentioned.  In  every  case  attention  must  be  paid  to  the 
character  of  the  load  as  regards  average  amount  and  con- 
stancy in  the  choice  of  an  appropriate  engine  for  the  work. 
In  cases  where  the  variations  of  load  are  likely  to  be  very 
sudden,  great  mechanical  strength  of  all  the  moving  parts  is 


-  100 


PER  CENT  LOAD  I.H.R. 
FIG.  171. 


absolutely  necessary,  and  an  attempt  should  be  made  in  plan- 
ning the  power  station  to  arrange  the  engine  for  its  best 
economy  at  average  load  as  nearly  as  this  can  be  predicted. 

With  care  in  planning  an  electric  power  station  the  engines 
can  be  made  to  give  an  exceedingly  good  performance,  much 
better  than  was  considered  possible  a  few  years  ago.  Fig. 
172  shows  a  set  of  curves  from  the  experiments  of  Prof.  R.  C. 
Carpenter  giving  the  performance  of  engines  of  different  kinds 


302 


ELKCTRIC   TRANSMISSION  OF  POWER. 


over  a  wide  range  of  loads,  from  mere  friction  load  up  to  50 
per  cent,  overload.  The  results  are  in  pounds  of  water 
evaporated  per  indicated  HPH.  The  immense  advantage  to 
be  gained  by  using  compound  and  triple-expansion  condensing 
engines  appears  plainly  from  the  curves.  Another  conspicu- 
ous fact  is  the  great  economy  attained  by  such  engines  over  a 
wide  range  of  load.  It  is  a  common  fallacy  to  suppose  that 


o.io 


1.50 


0.25  0.50  0.75  1.0  1.25 

PROPORTION  THAT  ACTUAL  LOAD  BEARS  TO  RATED  POWER 
FIG.   172. 

while  compound  or  triple-expansion  condensing  engines  are  all 
well  enough  at  steady  load,  simple  engines  have  the  advantage 
if  the  load  varies  over  a  wide  range.  The  facts  in  the  case  as 
shown  in  Fig.  172  are  exactly  the  reverse:  not  only  do  the  high 
expansion  engines  have  the  advantage  of  the  simple  engines 
at  their  rated  loads,  but  at  all  loads,  and  particularly  light  ones. 
And  their  advantage  is  so  great  that  under  any  ordinary  cir- 
cumstances the  use  of  a  simple  or  a  non-condensing  engine  for 


ENGINES  AND  BOILERS.  3°3 

power  generation  is  willful  waste  of  money.  If  the  saving  in 
first  cost  were  great  the  mistake  might  be  excusable,  but  the 
greater  amount  of  steam  required  for  running  simple  engines 
means  larger  boiler  capacity,  which  nearly  offsets  the  lower 
cost  of  engine.  For  example,  a  glance  at  Fig.  172  shows  that 
a  triple-expansion  condensing  engine  requires  only  half  the 
boiler  capacity  demanded  by  a  non-condensing  automatic  engine 
for  the  same  output.  In  other  words,  if  the  former  requires 
500  HP  in  boilers,  the  latter  will  need  1000  HP  in  boilers  for 
exactly  the  same  service.  And  the  same  holds  true  of  the 
capacity  of  the  stack,  feed-pumps,  steam-piping,  water-piping, 
and,  to  a  certain  extent,  even  of  the  building,  so  that  it  is 
almost  always  poor  economy  to  buy  a  cheap  type  of  engine. 

Many  blunders  are  made  by  being  too  hasty  in  buying 
engines  for  electric  service,  and  not  sufficiently  studying  the 
problem.  For  uniform  loads  the  selection  of  the  engines  can 
be  made  easily.  For  variable  loads  it  requires  great  astute- 
ness and  experience,  nor  is  it  safe  to  argue  from  experience 
based  on  other  kinds  of  variable  service.  No  engines  can  be 
subject  to  greater  variations  of  load  than  are  met  in  marine 
engines  driving  a  ship  in  a  high  sea.  If  the  screw  rises  from 
the  water  the  whole  load  is  thrown  off,  and  resumed  again  with 
terrible  violence  when  the  screw  is  submerged.  Nevertheless 
an  engine,  which  is  so  arranged  as  to  perform  well  under  these 
trying  circumstances,  might  perform  badly  when  put  on 
electric  railway  or  power  service,  not  because  of  its  inability 
to  stand  the  far  less  severe  changes  of  load,  but  for  the  reason 
that  the  average  load  would  be  much  further  from  its  full 
capacity  than  in  the  case  of  marine  practice.  For  large  rail- 
way and  power  service  it  is  best  to  use  direct  connected  units 
for  the  sake  of  compactness  and  economy.  If  a  station  is  of 
sufficient  magnitude  to  employ  four  or  five  500  HP  engines, 
direct  connecting  is  advisable  in  nearly  every  case. 

It  has  been  said  that  such  a  plant  has  a  lack  of  flexibility 
that  is  dangerous  in  case  of  sudden  and  great  variations  of 
load.  This  is  not  true  if  the  engines  have  been  intelligently 
proportioned  for  the  work  they  have  to  do,  although  in  some 
cases  there  has  been  trouble  due  to  the  fact  that  the  engines 
were  ill-fitted  to  operate  successfully  under  the  changes  of 
load  to  which  they  were  subjected.  As  a  matter  of  economy 


ELECTRIC    TRANSMISSION  OF  POWER. 


both  in  engines  and  dynamos,  it  is  desirable  to  work  direct 
coupled  plants  at  a  fairly  high  speed.  There  is  no  need  of  ex- 
aggerating the  size  of  both  engine  and  dynamo  for  the  sake  of 
running  at  50  to  70  revolutions  per  minute,  when  equally  good 
engines  and  dynamos  of  smaller  size  and  less  weight  can  be 
obtained  by  running  at  100  or  120  revolutions  or  more.  Much 
of  the  unwieldiness  charged  against  large  direct-coupled  units 
has  been  the  result  of  yielding  to  the  importunities  of  some 
engine  builder,  who  wanted  to  sell  a  very  large  machine,  and 
putting  in  an  engine  and  dynamo  working  at  absurdly  and  un- 
necessarily low  speed. 

Electric  power  transmission,  with  a  steam  engine  as  the 
prime  mover,  is  most  likely  to  be  developed  in  the  direction  of 
very  large  plants,  to  which  these  remarks  apply  most  forcibly, 
particularly  as  in  order  to  make  transmission  of  power  from 
a  steam-operated  station  profitable,  it  is  necessary  to  seek  the 
very  highest  efficiency.  Apart  from  the  cost  and  inconvenience 
of  very  low  speed  units,  it  must  be  borne  in  mind  that  the 


•  FIG.  173. 

mechanical  efficiency  of  large  low  speed  engines  with  heavy 
pistons  and  enormous  fly-wheels  is  lower  than  that  of  those 
designed  for  more  reasonable  speeds,  which  gives  added  reason 
for  moderation  in  planning  direct-coupled  units. 

Throughout  the  design  of  a  power  station  the  probability  of 
light  loads  must  be  considered.  Not  only  does  this  have  an 
important  bearing  on  the  economy  of  the  engines,  but  it  influ- 
ences that  of  the  boilers  as  well.  The  cost  of  operation  de- 
pends on  the  coal  consumption,  and  this  in  turn  not  only  on 
the  amount  of  steam  that  must  be  produced,  but  on  the  effi- 
ciency of  its  production. 


ENGINES  AND   BOILERS. 


305 


There  is,  however,  no  classification  of  boilers  on  which  one 
can  safely  rest  in  judging  of  their  economy.  There  is  much 
more  difference  in  economy  between  a  carefully  fired  and  a 


FIG.  174. 

badly  fired  boiler  of  the  same  kind,  than  there  is  between  the 
best  and  the  worst  type  of  boiler  in  ordinary  use.  Boilers  may 
be  generally  divided  into  three  classes:  Shell  boilers,  in  which 
the  water  is  contained  in  a  plain  cylindrical  tank  heated 
on  the  outside;  tubular  boilers,  in  which  there  are  one  or 


306 


ELECTRIC   TRANSMISSION  OF  POWER. 


many  tubes  running  lengthwise  of  the  boiler  shell,  and  serving 
as  channels  for  the  heated  gases  from  the  fire;  and  water-tube 
boilers,  in  which  the  water  is  contained  in  a  group  of  metallic 
tubes,  around  which  the  heat  of  the  fire  freely  plays.  Fig.  173 
shows  a  cross-section  through  the  furnace  of  a  bank  of  boilers 
of  the  first  class.  In  this  case,  three  shells  were  placed  over 
each  furnace,  communicating  with  a  common  steam  drum. 
Each  shell  was  30"  in  diameter  and  30'  long.  Fig.  174  rep- 
resents one  of  the  many  forms  of  tubular  boiler.  In  this  the 
structure  is  vertical,  with  a  furnace  at  the  bottom,  and  the 


FIG.  175. 

tubes  are  numerous  and  rather  small,  giving  a  large  heating- 
surface.  Tubular  boilers  are  very  often  arranged  horizontally, 
and  in  one  very  excellent  and  common  type  (return  tubular), 
the  flame  and  heated  gases  pass  horizontally  under  the  boiler 
shell  and  then  back  through  the  tubes  to  the  furnace  end 
and  thence  upward  into  the  stack.  A  typical  water-tube 
boiler  is  shown  in  Fig.  175.  Here  the  furnace  is  at  the  left 
of  the  cut  and  the  stack  at  the  right.  The  tubes  are  inclined  as 
is  usual  in  water-tube  boilers,  and  steam  space  is  secured  by 
the  drum  above.  Each  class  of  boiler  has  nearly  as  many 
modifications  as  there  are  makers,  most  of  them  being  with 
relation  to  the  arrangement  of  the  fire  with  respect  to  the 
boiler  proper. 


ENGINES  AND   BOILERS.  3°7 

As  to  the  merits  of  the  different  classes,  opinions  differ  very 
widely.  It  is  clear  from  experience  that  the  simple  shell 
boiler  is  decidedly  inferior  to  either  of  the  others  in  econ- 
omy, in  spite  of  its  simplicity  and  cheapness.  Of  late  years  it 
has  been  the  fashion  to  employ  water-tube  boilers  under  all 
sorts  of  conditions,  on  account  of  their  supposed  great  effi- 
ciency as  steam  producers,  safety  and  compactness.  Purely 
experimental  runs  with  such  boilers  often  show  phenomenal 
efficiency,  but  tests  under  working  conditions  sometimes  re- 
sult otherwise.  It  is  important  to  note  that  not  only  does 
skill  in  firing  produce  a  great  improvement  in  boiler  economy, 
but  that  by  influencing  the  firing  different  kinds  of  coal  give 
very  different  results  quite  independent  of  their  theoretical 
value  as  fuel.  The  thermal  value  of  coal,  or  other  solid  fuel, 
is  almost  directly  as  the  proportion  of  carbon  contained  in  it, 
and  for  comparative  purposes  boiler  tests  are  generally  re- 
duced to  evaporation  of  water  from  and  at  2i2Q  F.  per  Ib. 
of  combustible  used,  /.  <?.,  per  Ib.  of  carbon.  However,  the 
firing  in  different  furnaces  is  differently  affected  by  changes  in 
fuel,  so  that  it  is  impossible  to  predict  by  tests  on  one  boiler 
what  a  similar  one  will  do  under  other  conditions. 

Altogether,  the  subject  of  boiler  efficiency  is  a  difficult  and 
tangled  one,  since  the  conditions  are  constantly  changing, 
and  the  best  guide  is  found  in  the  general  result  of  a  long  series 
of  tests  rather  than  in  theories  of  combustion.  Forcing  the 
output  of  a  boiler  usually  injures  its  efficiency  by  compelling 
the  combustion  of  an  abnormal  amount  of  coal  for  the  grate 
surface  of  the  furnace.  It  follows  that  a  boiler  is  apt  to  be 
more  efficient  at  moderate  loads  than  at  very  high  ones.  As 
this  is  the  reverse  of  what  happens  in  an  engine,  it  might  be 
supposed  that  the  boiler  could  partially  compensate  for  engine 
inefficiency,  but  on  the  contrary  inefficient  production  of  steam 
is  anywhere  and  always  a  bad  thing  in  itself,  and  only  to  be  tol- 
erated for  some  very  good  reason.  In  marine  practice,  boilers 
may  sometimes  have  to  be  forced  to  a  high  output  to  save 
weight  and  space.  In  electric  stations  it  is  sometimes  better 
to  force  the  boilers  at  the  hours  of  heavy  load,  than  to  keep  a 
relay  of  boilers  banked  in  readiness  for  use,  but  except  for 
this,  the  boilers,  like  the  rest  of  the  plant,  should  be  worked 
as  near  their  maximum  efficiency  as  possible. 


308 


ELECTRIC   TRANSMISSION  OF  POWER. 


The  best  fuel  to  use  is  not  at  all  invariably  that  of  the 
highest  thermal  value,  in  fact  with  the  proper  furnace  a  grade 
of  coal  only  moderately  good  is  very  often  the  most  economi- 
cal. In  starting  a  steam  plant  of  any  kind  comparative  tests 
of  various  coals  should  generally  be  made,  and  are  more  than 
likely  to  pay  for  themselves  many  times  over.  In  absolute 
heating  value  various  kinds  of  fuel  compare  about  as  follows: 


KIND  OF  FUEL. 

HEAT  OF  COMBUSTION. 

EVAPORATION. 

Best  anthracite          .  .           

je  2^0 

TC   R 

Id.  5OO 

TC  o 

14  37^ 

Id  Q 

Cumberland           ...                  . 

I  a  7co 

J.q..y 
•tA    n 

12  7^O 

\"\  2 

12  5OO 

TO    O 

Lignite  

II  750 

12  2 

Peat  dry  

Q  6^O 

IO  O 

Wood,  dry  

7  2^O 

7e 

The  heat  of  combustion  is  per  Ib.  of  fuel,  and  is  given  in 
thermal  units,  this  unit  being  the  heat  required  to  raise  i  Ib. 
of  water  i°  F. 

The  evaporation  gives  the  pounds  of  water  which  can  be 
evaporated  from  and  at  212°  F.  by  the  complete  utilization 
of  the  annexed  heats  of  combustion.  In  other  words  no  more 
than  15  Ibs.  of  water  can  possibly  be  evaporated  by  i  Ib. 
of  coal  of  the  thermal  value  of  14,500.  Extravagant  claims 
are  sometimes  made  for  patented  boilers  of  strange  and 
unusual  kinds,  sa  it  is  well  to  bear  these  figures  in  mind  and 
to  remember  that  you  cannot  evaporate  more  water  than 
the  figures  indicate,  any  more  than  you  can  draw  a  gallon 
out  of  a  quart  bottle.  In  practice  coal  is  likely  to  fall  perhaps 
10  per  cent,  below  the  thermal  values  given  above.  Good 
boilers  with  careful  firing  will  utilize  from  70  to  75  per  cent, 
of  the  thermal  value  of  the  coal.  Occasional  experimental 
runs  may  give  slightly  higher  figures,  but  only  under  very 
exceptional  circumstances. 

Now  as  to  actual  tests  under  boilers.  Examinations  of 
more  than  a  hundred  carefully  conducted  tests  by  various 
authorities  show  from  8  to  13  Ibs.  of  water  evaporated 
from  and  at  212°  per  Ib.  of  combustible.  As  average  good 


ENGINES  AND   BOILERS. 


3<>9 


steam  coal  contains  from  8  to  15  per  cent,  of  ash  and  moist- 
ure, these  results  correspond  to  from  7  to  n^  Ibs  of  water 
per  Ib.  of  coal.  Now  and  then  a  single  test  gives 
a  result  a  few  hundredths  of  a  pound  better  than  13  Ibs. 
per  Ib.  of  combustible,  and  an  occasional  poor  boiler  shows 
less  than  8  Ibs.  Generally  from  10  to  15  Ibs.  of  coal  are  con- 
sumed per  hour  per  square  foot  of  grate  surface.  The  fol- 
lowing table  gives  a  general  idea  of  the  results  of  boiler  tests, 
good,  bad,  and  indifferent. 


KIND  OF  BOILER. 

KIND  OF  COAL. 

EVAPORATION. 

Return  tubular             ...            . 

Welsh  steam                    .        . 

J  -J     TO 

Water-tube  

j  Bituminous,    3    parts  ) 

13  01 

Return  tubular             

(  pea  and  dust,  I  part  )  '  '  '  ' 
Cumberland 

12  4.7 

Vertical       "       

12  2Q 

Return        "       

it 

12  O7 

« 

12  O3 

tt             «< 

Anthracite           

II  63 

Marine  

1  1.  4.4. 

Anthracite                                 . 

n-JT 

Cumberland      ..        

IO  98 

Anthracite     

10  88 

Water-tube 

Cumberland 

IO  7Q 

Marine                                 . 

Welsh  steam           .  .       .        . 

IO  4.4. 

Return  tubular  

Anthracite       

Locomotive 

Coke 

jo  39 

Water-tube.            

Anthracite                      . 

io  oo 

9CC 

14 

9.22 

f~>           ,          ,           , 

Q    -* 

8.44 


The  evaporation  is  per  Ib.  of  combustible.  The  most  strik- 
ing feature  of  this  table  is  the  small  difference  in  efficiency 
between  the  various  kinds  of  boiler.  Putting  aside  the  cylin- 
drical shell  boilers,  which  are  distinctly  inferior  to  the  others, 
it  appears  that  as  to  the  other  types  of  boiler  there  is  little  to 
choose  on  the  score  of  economy.  The  difference  between  the 
better  and  worse  boilers  of  each  class,  due  to  difference  of 
design,  condition,  and  firing,  is  much  greater  than  the  differ- 
ence between  anv  two  classes.  Even  the  same  boiler  with 
different  fuels,  firing,  or  when  in  different  condition,  may  give 
evaporative  results  varying  by  30  per  cent.  Economy  de- 
pends vastly  more  on  careful  firing  and  proper  proportion- 


310  ELECTRIC    TRANSMISSION  OF  POWER. 

ing  of  the  grate  and  he,ating  surfaces  to  the  fuel  used,  than 
upon  the  kind  of  boiler.  In  fact,  judging  from  all  the  available 
tests  the  differences  between  various  types  of  boiler  when 
properly  proportioned  are  quite  small. 

The  most  that  can  be  said  is  that  plain  shell  boilers  are  some- 
what inferior  to  the  other  forms,  of  which  the  horizontal  return 
tubular  and  the  water-tube  have  given  slightly  higher  results 
than  the  others.  Water-tube  boilers  are  generally  rather 
compacter  and  stand  forcing  better  than  ordinary  tubular  boil- 
ers. They  also  are  less  likely  to  produce  disastrous  results  if 
they  explode.  On  the  other  hand,  they  are  more  expensive, 
and  are  as  a  class  hard  to  keep  in  good  condition,  particularly 
if  the  water  supply  is  not  of  good  quality. 

Probably  under  average  conditions  a  well-designed  horizon- 
tal return  tubular  boiler  will  give  as  great  evaporative  effi- 
ciency as  can  regularly  be  attained  in  service,  and  the  choice 
between  it  and  a  water-tube  boiler  is  chiefly  in  economy  of 
space  and  capacity  for  forcing.  There  is  no  excuse  for  the 
explosion  of  any  properly  cared  for  boiler. 

The  actual  evaporation  secured  per  Ib.  of  total  fuel  is 
something  quite  different  from  the  figures  in  the  table  just 
given.  In  the  first  place  allowance  must  be  made  for  ash  and 
fuel  used  for  banking  the  fires.  In  the  second  place  in  regular 
running  the  firing  is  seldom  as  careful  as  in  tests. 

On  account  of  these  the  evaporations  per  Ib.  of  combustible 
given  in  the  table  must  be  reduced  from  15  to  20  per  cent,  to 
correct  the  result  to  pounds  of  coal  used  in  actual  service. 

Ten  Ibs.  of  water  or  over  evaporated  from  and  at  212°  per  Ib. 
of  total  fuel  may  be  regarded  as  exceptionally  good  practice  in 
every-day  work.  Nine  to  10  Ibs.  under  the  same  conditions 
represents  fine  average  results,  and  8  to  9  Ibs.  is  much  more 
common.  In  fact  8  Ibs.  is  an  unpleasantly  frequent  figure, 
particularly  in  boilers  operated  under  variable  Load,  such  as  is 
generally  found  in  electric  plants  of  moderate  size.  All  these 
facts  point  out  the  necessity  of  thorough  and  careful  work  in 
every  part  of  a  power  plant.  Bad  design  or  careless  opera- 
tion  anywhere  plays  havoc  with  economy.  In  most  instances 
far  too  little  attention  is  paid  to  the  adaptation  of  the  furnace  to 
the  particular  fuel  used.  In  case  of  attempting  power  trans- 
mission from  cheap  coal  at  or  near  the  mines,  the  furnace  and 


ENGINES  AND   BOILERS. 


firing  problem  is  of  fundamental  importance.  Most  furnaces 
are  constructed  to  meet  the  requirements  of  high-grade  fuel 
and  are  quite  likely  to  work  badly  with  anything  else.  In 
transmitting  power  from  cheap  coal  the  grate  surface,  draft 
and  so  forth  must  be  carefully  arranged  with  reference  to  the 
grade  of  fuel  to  be  used  and  not  with  reference  to  standard 
coals  used  elsewhere.  The  methods  of  firing,  too,  require 
careful  attention. 

At  the  present  time  mechanical  stokers  are  in  very  extensive 
use  in  some  parts  of  the  country.  The  reports  from  them  are 
of  very  varying  nature,  but  the  consensus  of  opinion  seems  to 
be  that  they  are  advantageous  in  working  medium  and  low-grade 
coals,  but  of  doubtful  utility  in  the  case  of  high-grade  coals. 
They  are  somewhat  expensive  and  require  intelligent  care 
now  and  then  like  all  other  machinery,  but  when  it  comes  to 
firing  large  amounts  of  cheap  fuel  at  a  fairly  regular  rate  they 
do  most  excellent  work.  When  coal  is  dear  careful  hand 
firing  is  probably  more  economical  than  any  mechanical 
method.  With  first-class  coal  and  boilers  one  good  fireman 
and  a  coal-passer  can  take  care  of  2000  KW  in  modern  appara- 
tus, so  that  the  total  cost  of  firing  is  not  a  very  serious  matter. 
A  poor  fireman  is  dear  at  any  price,  and  quite  as  disadvantage- 
ous to  the  station  as  a  poor  engineer. 


KIND  OF  ENGINE. 

COAL  PEI 

i  IHP  HOUR. 

CONDENSING. 

NON-  CONDENSING. 

Simple  dependent  valve           .  .                   . 

2  77 

3  66 

"        independent  valve    

2.T* 

ail 

Compound   dependent  valve 

2  22 

2  66 

independent  valve             

2  OO 

2.44 

Triple  dependent  valve.          

1.88 

"        independent  valve 

i  66 

"                "                "     lanre 

1.44 

Reverting  now  to  engine  performances,  we  may  form  a  fairly 
definite  idea  of  what  may  be  expected  in  the  way  of  coal  con- 
sumption per  indicated  horse  power  hour. 

The  foregoing  table  shows  the  coal  consumption  of  the 
various  kinds  of  engines,  based  on  the  burning  of  i  Ib.  of 
coal  for  each  9  Ibs.  of  feed  water  used.  Although  greater 


312  ELECTRIC   TRANSMISSION  OF  POWER. 

evaporation  can  often  be  obtained,  9  Ibs.  of  water  per  Ib. 
of  coal  is  a  very  good  performance  indeed,  decidedly  better 
than  is  found  in  general  experience.  It  presupposes  good 
boilers,  good  coal,  and  skillful  firing,  such  as  one  has  a  right 
to  expect  in  a  large  power  plant. 

The  figures  apply  only  to  engines  of  several  hundred  HP, 
at  or  near  their  points  of  maximum  economy  and  operated 
from  a  first-class  boiler  plant. 

They  can  be  and  are  reached  in  regular  working,  and  are 
sometimes  exceeded.  A  combination  of  great  efficiency  at 
the  boilers  and  small  steam  consumption  in  the  engine  some- 
times gives  remarkable  results.  The  best  triple-expansion 
condensing  engines  worked  under  favorable  conditions  can  be 
counted  on  to  do  a  little  better  than  1.5  Ibs.  of  coal  per 
IHP  hour,  occasionally  even  in  the  neighborhood  of  1.25 
Ibs.  Even  with  compound  condensing  engines  tests  are 
now  and  then  recorded,  showing  below  1.75  Ibs.  of  coal 
per  IHP  hour.  But  these  very  low  figures  are  the  result  of 
the  concurrence  of  divers  very  favorable  conditions,  and  those 
just  tabulated  are  as  good  as  one  really  has  the  right  to 
expect.  It  must  not  be  supposed  that  the  weight  of  coal  used 
per  HP  hour  nece-ssarily  determines  the  economy  of  the 
plant.  The  cost  of  fuel  of  course  varies  greatly,  and  its  price 
in  the  market  is  by  no  means  proportional  to  its  thermal  value. 
As  a  rule  the  coals  which  give  the  best  economic  results  are 
not  those  of  thd  greatest  intrinsic  heating  power.  On  the 
contrary,  dollar  for  dollar,  the  best  results  are  very  frequently 
obtained  from  cheap  coal,  or  mixtures  of  inferior  coal  with 
a  portion  of  a  better  grade.  Hence  the  boilers  of  a  plant 
which  is  a  model  of  economy  may  show  an  evaporation  of  only 
7  or  8  Ibs.  of  water  per  Ib.  of  coal.  Boiler  tests  with  the  con- 
ditions of  economy  in  view  are  of  great  importance  and  are 
likely  to  pay  for  themselves  tenfold  in  even  a  few  months 
of  operation. 

A  word  here  about  fuel  oil.  Petroleum  has,  weight  for 
weight,  much  greater  heating  power  than  coal.  Its  heat  of 
combustion  is  about  20,000  to  21,000  thermal  units,  it  costs 
little  to  handle  and  fire,  leaves  no  ash  and  refuse  to  be  taken 
care  of,  produces  little  smoke,  and  is  generally  cleanly  and 
convenient. 


ENGINES  AND   BOILERS.  313 

It  has  been  thoroughly  tried  by  some  of  the  largest  elec- 
trical companies  in  this  country,  and  at  moderate  prices,  a 
dollar  a  barrel  or  less,  is  capable  of  competing  on  fairly  even 
terms  with  coal.  But  experience  has  shown  some  curious 
facts  about  its  performance.  The  amount  of  steam  or  equiva- 
lent power  required  to  inject  and  vaporize  the  oil  in  one  of 
the  most  skillfully  handled  plants  in  existence  amounts  to  no 
less  than  7^  per  cent,  of  the  total  steam  produced.  And 
curiously  enough,  the  cost  of  oil  for  firing  up  a  fresh  boiler, 
and  the  time  consumed,  compare  unfavorably  with  the  results 
obtained  from  coal.  In  spite  of  the  great  amount  of  heat 
evolved  from  fuel  oil,  it  appears  to  be  less  effective  than  coal 
in  giving  up  this  heat  to  the  boiler  by  radiation  and  convection. 
There  is  good  reason  to  believe  that  more  than  half  the  total 
heat  of  combustion  of  incandescent  fuel  is  given  off  as  radiant 
heat,  and  most  of  the  remainder  is  of  course  transferred  by 
convection  both  of  heated  particles  of  carbon  and  of  molecules 
of  gas. 

It  is  not  unlikely,  therefore,  that  a  petroleum  fire  with  its 
small  radiating  power  and  comparative  absence  of  .incan- 
descent particles,  fails  in  economy  through  inability  to  give 
up  its  heat  readily.  This  view  of  .the  case  is  borne  out  by  the 
facts  above  cited  and  by  the  abnormally  high  temperature  of 
the  escaping  gases  often  found  in  boiler  tests  with  petroleum 
fuel.  At  all  events  it  is  clear  from  such  tests  that  the  evapo- 
ration obtained  from  fuel  oil  is  not  so  great  as  would  be 
expected  from  its  immense  heat  of  combustion,  and  unless 
at  an  exceptionally  low  price,  its  use  is  less  economical  than 
that  of  coal. 

The  most  striking  innovation  of  recent  years  in  the  genera- 
tion of  mechanical  power  by  steam  is  the  development  of  the 
steam  turbine.  Year  by  year  during  the  past  decade  it  has 
slowly  grown  from  experiment  to  realization  until  at  the  pres- 
ent time  it  has  reached  a  position  that  demands  for  it  most 
serious  consideration.  It  looks  very  much  as  if,  for  many 
purposes,  the  reciprocating  steam  engine  would  be  hard 
pushed. 

Strangely  enough  the  steam  turbine  or  impulse  wheel  is  the 
earliest  recorded  form  of  steam  engine,  dating  clear  back  to 
Hero  of  Alexandria,  who  flourished  about  130  B.  c.  The  engine 


ELECTRIC    TRANSMISSION  OF  POWER. 

which  Hero  suggested  was  merely  a  philosophical  toy,  and  it 
took  nineteen  centuries  beyond  his  day  to  produce  any  engine 
that  was  not  a  toy,  but  now  after  two  thousand  years  Hero's 
idea  has  borne  fruit. 

The  fundamental  principle  of  the  steam  turbine  is  just  that 
of  the  water  turbine — directing  fluid  pressure  against  a  series 
of  rotating  buckets.  The  first  practical  steam  turbine,  devised 
by  De  Laval,  is  very  closely  akin  to  the  Pelton  water  wheel 
and  to  the  little  water  motors  sometimes  attached  to  faucets 
for  furnishing  a  small  amount  of  power.  The  essential  fea- 
tures of  his  apparatus  are  well  shown  in  Fig.  176.  It  consists 


FIG.   176. 

of  a  narrow  wheel  A  with  buckets  around  its  periphery,  re- 
volving within  a  housing  B  and  supported  by  a  rather  long  and 
slender  shaft.  Bearing  upon  the  buckets  at  an  acute  lateral 
angle  is  the  steam  jet  E,  "in  this  case  one  of  three  equidistant 
jets  playing  on  the  same  wheel.  To  obtain  the  most  efficient 
working  of  the  jet  the  steam  nozzle  is  somewhat  contracted  at 
Z>,  a  little  way  back  from  the  buckets.  The  steam  is  discharged 
on  the  other  side  of  the  wheel  as  shown.  It  strikes  the  buckets 
as  a  jet  at  great  velocity,  and  should,  if  the  conditions  were 
just  right,  expend  nearly  all  its  energy  in  driving  the  wheel  and 
should  itself  leave  it  at  or  near  zero  velocity.  Of  course  this 
condition  does  not  hold  in  practice,  but  stil!  a  steam  turbine  of 


ENGINES  AND   BOILERS.  315 

this  De  Laval  construction  is  capable  of  doing  marvelously  well. 
The  main  objection  to  this  form  is  the  enormously  high  rotative 
speeds  necessary  for  efficient  running.  Here  as  in  hydraulic 
impulse  wheels  the  peripheral  velocity  should  be  one-half  the 
spouting  velocity  of  the  fluid.  With  high  pressure  steam  this  is, 
when  one  works  the  turbine  condensing,  3000  to  5000  feet  per 
second.  In  practice  these  De  Laval  wheels  have  usually  been 
geared  down  to  a  driving  shaft,  but  the  wheel  itself  has  run  at 
10,000  to  30,000  r.p.m.,  seldom  below  the  former  figure  even  in 
large  sizes.  But  the  economy  reached  has  sometimes  been  very 
high,  as  in  some  tests  about  a  year  ago  in  France,  when,  with  an 
initial  pressure  of  192  Ibs.,  a  300  HP  turbine  showed  a  steam 


I 


cccccccc 


CCCCCCXCC 


StatlonarytBlades 

Moving  Blades 


Moving  Blades 


FIG.  177. 

consumption  of  only  13.92  Ibs.  per  effective  HPH — a  figure 
seldom  reached  even  with  triple-expansion  engines.  The 
governing  is  by  throttling  the  steam  supply  in  response  to  the 
movement  of  a  fly-ball  governor  of  the  kind  generally  familiar. 
The  inconvenience  of  the  very  high  rotative  speed  of  such 
turbines  has  led  to  the  development  of  forms  working  more 
along  the  lines  of  hydraulic  turbines,  of  which  by  far  the  best 
known  is  the  Parsons  turbine,  which  has  recently  made  so 
striking  a  record  in  marine  work,  having  been  applied  to  sev- 
eral British  torpedo-boats.  In  this  remarkable  machine  the 
passage  of  the  steam  is  parallel  to  the  axis  of  rotation  instead 
of  tangential,  and  its  hydraulic  prototype  is  the  parallel- 
flow  turbine,  shown  in  diagram  in  Fig.  183.  In  fact  the  steam 
is  passed  successively  through  a  large  number  of  such  parallel- 
flow  turbines,  gradually  expanding  and  giving  up  its  energy  to 
the  successive  runners  located,  of  course,  on  the  same  shaft 
The  course  of  the  expanding  steam  is  well  shown  in  Fig.  177, 


316  ELECTRIC    TRANSMISSION  OF  POWER. 

which  gives  in  diagram  its  progression   through  four  sets  of 
vanes,  two,  i  and  3,  being  rings  of  guide-blades,  and  the  others, 


2  and  4,  rings  of  runner  blades.     The  steam,  starting  at  pres- 
sure P,    expands  successively  to  P^  P^  Pm,  Piv,  expanding 


ENGINES  AND  BOILERS.  317 

sharply  against  the  runner  blades  and  giving  them  a  reactive 
kick  as  it  leaves.  In  this  case  the  stream  velocity  against,  say 
the  runner  blades  2,  is  not,  as  in  the  De  Laval  form,  the  full 
spouting  velocity  due  to  the  initial  head  of  steam,  but  merely 
that  corresponding  to  the  differential  pressure  P-PV  This 
enables  the  peripheral  speed  of  the  runner  to  be  kept  within 
reasonable  limits  without  violating  the  conditions  of  economy, 
but  the  turbine  at  best  is  not  a  slow-speed  machine. 

Fig.  178  is  a  longitudinal  section  through  the  Parsons 
type  of  steam  turbine  as  developed  in  this  country  by  the 
Westinghouse  Machine  Co.  The  steam  enters  from  the  supply 
pipe  controlled  by  the  governor  and  comes  first  into  the  annu- 
lar chamber  A  at  the  extreme  left-hand  end  of  the  runner. 
The  runner  blades  are  graduated  in  size  so  that  the  expanding 
steam  may  give  nearly  a  uniform  useful  pressure  per  blade, 
and  to  this  end  the  diameter  of  the  runner  hub  is  twice  in- 
creased as  the  steam  expands  towards  the  exhaust  chamber  B. 
The  endwise  thrust  of  the  steam  entering  the  turbine  from  A 
is  balanced  by  its  equal  pressure  on  the  balancing  piston  C, 
which  revolves  with  the  runner.  To  the  left  of  this  is  an 
annular  steam  space  and  a  second  balance  piston  C.  Now 
when  the  expanding  steam  has  passed  the  first  section  of  the 
runner  into  the  steam  space  E  it  can  flow  back  through  the 
channel  /''and  the  post  D  against  this  second  balance  piston. 
Still  further  to  the  left  is  a  second  steam  space  and  a  third 
piston  C,  which  is  similarly  exposed  to  the  pressure  from  G, 
where  the  third  stage  of  expansion  begins.  The  effect  of  this 
balancing  system  is  to  render  the  end  thrust  negligible  what- 
ever may  be  the  ratio  of  expansion  in  the  turbine.  A  thrust 
bearing  at  H  keeps  the  working  parts  positioned  and  takes  up 
the  trivial  thrusts  which  may  incidentally  be  present.  J,  J,  J 
are  the  bearings  which  are  out  of  the  ordinary  in  that  within 
the  gun-metal  sleeve  that  forms  the  bearing  proper  are  three 
concentric  sleeves  fitting  loosely.  The  clearance  between 
them  fills  with  oil,  cushioning  the  bearings.  Now  if  the  run- 
ner is  not  absolutely  in  balance  there  is  a  certain  flexibility  in 
the  bearings  so  that  the  runner  can  rotate  about  its  center  of 
gravity  instead  of  its  geometrical  center,  thus  stopping 
vibration.  An  equivalent  expedient  is  found  in  the  De  Laval 
turbine,  the  shaft  of  which  is  deliberately  made  slightly  flexible 


ELECTRIC    TRANSMISSION  OF  POWER. 


so  that  it  may  take  up  rotation  about  its  center  of  gravity. 
K  is  a  pipe  which  again  takes  up  the  work  of  keeping  the 
pressure  balanced  by  connecting  the  exhaust  chamber  B  with 
the  steam  space  behind  the  last  balance  piston.  At  M  is  a 
simple  oil  pump  taking  oil  from  the  drip  tank  N  and  lifting  it 
to  the  tank  O,  whence  it  is  distributed  to  the  bearings.  A  by 
pass  valve  P  turns  high  pressure  steam  directly  into  the  steam 
space  E,  in  case  a  very  heavy  load  must  be  carried,  or  a  con- 
densing turbine  temporarily  run  non-condensing.  It  is  a 


WHEN  RUNNING  LIGHT  LOAD 

FIG.  179. 

flexible  coupling  for  the  driving  shaft,  and  at  that  point  too  is 
the  worm  gear  that  drives  the  governor.  The  governor  in 
its  operation  is  somewhat  peculiar.  Instead  of  throttling  the 
steam  supply  so  as  to  reduce  the  effective  pressure,  the  steam 
is  always  sent  to  the  turbine  at  full  boiler  pressure,  but  discon- 
tinuously.  The  main  steam  valve  is  controlled  by  a  little 
steam  relay  valve  which  is  given  a  regular  oscillatory  motion 
by  a  lever  driven  from  an  eccentric.  The  steam  is  thus  ad- 
mitted to  the  turbine  in  a  series  of  periodic  puffs.  Now  the 
fulcrum  of  this  valve  lever  is  movable  and  is  positioned  by 
the  fly-ball  governor,  so  that  the  position  of  the  valve  with  rela- 
tion to  the  port  is  varied  without  changing  the  rate  or  ampli- 
tude of  the  valve  motion.  Hence  the  length  of  the  puffs  is 
changed  so  that  while  at  full  load  steam  is  on  during  most  of 
the  period,  at  light  load  it  is  on  for  only  a  small  part  of  each 


ENGINES  AND   BOILERS. 


319 


period.  This  is  well  shown  graphically  by  Fig.  179,  which  is 
.self-explanatory. 

The  governor  balls  are  so  arranged  that  they  work  both 
ways,  their  mid-position  corresponding  to  full  admission  of 
steam,  so  that  a  violent  overload  can  be  made  to  shut  off  steam, 
and  a  break  in  the  governor  driving  gear  will  do  the  same 
instead  of  letting  the  turbine  run  away. 

These  turbines  are  capable  of  operating  with  really  remark- 
able efficiency.  Fig.  180,  from  the  makers'  tests,  gives  the 


25   50   75  100  125  150  175  200  225  250  275  300  325  350  375  400  425 

ELECTRICAL  HORSE  POWER 
FIG.  180. 

performance,  both  condensing  and  non-condensing,  of  a  Wes- 
tinghouse-Parsons  turbine  directly  coupled  to  a  300  KW 
quarter-phase  alternator  giving  440  volts  at  6o~,  the  speed  be- 
ing 3600  r.p.m.  Operated  condensing,  the  steam  consumption 
-at  full  load  falls  to  about  16.4  Ibs.  per  electrical  HPH,  and  is 
below  20  Ibs.  from  125  HP  up.  This  extraordinary  uniformity 
of  performance  at  large  and  small  loads  is  mainly  due  to  the 
very  small  frictional  losses  in  the  turbine,  although  it  is  helped, 
perhaps,  by  the  load  curve  of  the  generator.  The  results  when 
working  non-condensing  are  very  much  inferior  to  these,  rela- 
tively worse  than  in  an  ordinary  steam  engine. 


320  ELECTRIC   TRANSMISSION  OF  POWER. 

Altogether  it  is  a  wonderful  showing  for  the  steam  turbine. 
The  writer  believes  Fig.  180  to  be  entirely  trustworthy,  as  it 
corresponds  very  closely  with  certain  independent  tests  now 
in  his  possession,  from  another  turbine  of  the  same  capacity 
and  speed,  in  which  tests  the:. makers  of  the  turbine  had  no 
part.  The  substance  of  the  matter  is  that  the  modern  steam 
turbine  will  work  as  efficiently  as  a  first-class  compound  con- 
densing engine,  and  can  not  only  be  more  cheaply  made,  but 
takes  up  very  much  less  room.  In  the  same  way,  for  electri- 
cal purposes  a  directly  connected  generating  set  with  steam 
turbine  is,  or  ought  to  be,  much  cheaper  and  smaller  than 
those  now  in  use,  while  retaining  equally  high  efficiency. 
High  rotative  speed  is  by  far  the  cheapest  way  of  getting  out- 
put, and  when,  as  in  this  case,  no  heavy  reciprocating  parts 
are  involved,  there  is  no  good  reason  for  objecting  to  high 
speed.  The  present  fashion  for  low-speed  dynamos  is  largely 
a  fad  having  its  origin  in  direct  coupling  to  Corliss  engines, 
and  with  the  modern  stationary  armature  construction  there  is 
no  reason  why  high  rotative  speed  should  not  be  used,  at  least 
in  alternators. 

In  Plate  IX  is  shown  the  first  large  turbine-driven  generator 
installed  for  regular  commercial  service  in  this  country. 
Smaller  ones  have  been  in  use  in  isolated  plants  for  some  time, 
but  this  1500  KW  set,  recently  installed  for  the  Hartford 
Electric  Light  Co.  is  the  first  important  installation  of  this 
kind.  The  turbine  is  designed  for  a  maximum  output  of 
3000  HP  at  a  speed  of  1200  r. p.m.,  and  the  complete  r.et, 
weighing  only  175,000  Ibs.,  takes  a  floor  space  of  but 
33'  3"  x  8'  9".  The  generator  is  a  quarter-phase  machine  at 
6o~  frequency.  This  outfit  has  not  yet  been  in  operation 
long  enough  for  a  thorough  test  under  commercial  conditions, 
but  it  should  be  capable  of  giving  an  efficiency  rather  better 
than  that  shown  in  Fig.  180 — probably  a  little  over  14  Ibs.  of 
steam  per  electrical  HPH  at  steady  full  load;  in  other  words,  it 
should  do  nearly  or  quite  as  well  as  a  triple-expansion  engine. 

Alternators  may  be  conveniently  and  cheaply  built  for  the 
speed  implied  in  steam  turbine  practice,  but  continuous  cur- 
rent generators  involve  some  difficulties.  For  power  trans- 
mission work  turbine  generators  have  much  to  recommend 
them,  and  there  is  a  good  chance  for  their  taking  a  very  impor- 


PLATE    IX. 


ENGINES  AND   BOILERS. 


321 


tant  place  in  the  development  of  the  art.  There  has  not  yet 
been  accumulated  enough  experience  with  them  to  enable  a 
careful  judgment  of  their  practical  properties  to  be  formed, 
or  to  justify  an  unqualified  indorsement.  But  they  look 
exceedingly  promising. 

Considerable  space  has  here  been  given  to  describing  some 
of  the  details  of  such  machines,  in  the  belief  that  they  are  of 


8000 
g 

O 

1500 

S 

X 


1009- 


500 


NOC*  6 

FIG.   181. 

sufficient  importance  to  warrant  it.  They  certainly  have 
already  proved  their  right  to  a  place,  and  the  question  is  now 
merely  that  of  the  probable  limitations  of  their  usefulness, 
which  only  protracted  experience  can  disclose. 

For  an  electrical  power  station  operated  by  steam  power  the 
vital  economical  question  is  the  cost  of  fuel  per  kilowatt  hour, 
rather  than  the  performance  of  engines  and  boilers  alone. 
This  final  result  involves  the  performance  of  the  station  appa- 


322 


ELECTRIC    TRANSMISSION  OF  POWER. 


ratus  under  varying  loads,  too  frequently  rather  light,  and,, 
implicitly,  the  skill  of  the  operator  in  keeping  his  apparatus 
actually  running  as  near  its  point  of  maximum  economy  as 
possible  in  spite  of  changes  in  the  electrical  output.  This 
personal  element  forbids  a  reduction  of  the  facts  to  general 
laws,  but  a  concrete  example  will  be  of  service  in  showing;  what 
may  be  expected  in  a  well-designed  and  well-operated  power 
plant.  Fig.  181  shows  a  pair  of  "load  lines,"  from  a  large  and 
particularly  well-operated  power  plant.  The  solid  line  shows 
the  variations  of  load  throughout  a  day  in  the  latter  end  of 
January,  and  the  broken  line  the  variations  of  load  during  a 
day  early  in  April. 

The  early  darkness  of  a  winter's  day  is  very  obvious  in  the 


COST.  MILLS  PER  K.W.  HOUR 

£ 

\ 

\ 

\ 

\ 

\ 

/ 

s. 

**S*^ 

^ 

12  P.M. 


12  M. 

HOUR 
FIG.   182. 


12P.M. 


former  line.  The  station  carries  in  addition  to  lights  a  heavy 
motor  service  that  keeps  up  a  fairly  uniform  load  through- 
out the  day,  until  the  sudden  call  for  lights  in  the  early 
evening.  The  load  factor  shown  by  the  solid  line  is  .35  (/*.  <?., 
this  is  the  ratio  between  maximum  and  average  load). '  The 
second  load  line  gives  a  much  better  relation  between  these 
quantities,  the  load  factor  being  .64,  which  is  quite  usual  in 
this  station  during  the  spring  and  summer.  Of  course  every 
effort  is  exerted  to  keep  the  machines  which  are  in  use  as  fully 
loaded  as  possible.  In  spite  of  this  the  small  output  during 
the  early  morning  hours,  coupled  with  the  losses  due  to  circu- 
lating pumps  and  other  minor  machinery,  and  the  fuel  used  for 


ENGINES  AND  BOILERS.  323 

banking  and  starting  fires,  brings  the  cost  of  fuel  during  this 
period  far  above  the  average  for  the  day.  The  curve  in  Fig. 
182  shows  roughly  the  variation  in  the  cost  of  fuel  per  KW- 
hour  throughout  the  day,  taken  from  the  average  of  a  num- 
ber of  tests.  As  the  fuel  cost  in  a  large  central  station  is  a 
considerable  portion  of  the  total  expense,  it  is  evident  that  the 
result  is  an  excellent  one.  During  all  the  hours  of  heavy  load 
the  cost  of  fuel  is  less  than  six-tenths  of  a  cent  per  KW-hour, 
and  the  total  cost  of  production  but  little  more.  This  result 
will  give  an  excellent  idea  of  the  cost  of  generating  power  on 
a  large  scale  with  cheap  coal.  It  is,  however,  exceptionally 
good,  and  can  only  be  equaled  by  a  very  well  managed  plant 
with  the  best  modern  equipment  both  electrical  and  mechanical. 
Of  course  the  expenses  of  distribution,  administration 
and  the  like  must  be  taken  into  account  in  considering  the 
cost  per  KW-hour  delivered.  The  general  question  of  station 
expenses  cannot  be  here  investigated,  but  this  brief  digression 
gives  some  idea  of  the  necessary  relation  between  the  character 
of  the  work  and  the  commercial  results  in  generating  electric 
power  on  a  large  scale,  so  far  as  the  use  of  steam  engines  as 
prime  movers  is  concerned. 


CHAPTER  IX. 

WATER    WHEELS. 

THE  importance  of  the  development  of  water  powers  for 
electrical  purposes  we  have  already  come  fully  to  realize.  The 
lessons  of  the  last  few  years  have  been  exceedingly  valuable 
ones,  and  it  is  safe  to  say  that  the  utilization  of  water  powers 
for  electrical  transmission  will  be  kept  up  until  every  one 
which  is  capable  of  commercially  successful  development  is 
worked  to  its  utmost  capacity.  In  spite  of  the  length  of  time 
that  water  wheels  of  various  sorts  have  been  used,  it  is  only 
very  recently  that  these  prime  movers  have  been  brought  to  a 
stage  of  development  that  renders  them  satisfactory  for  elec- 
trical purposes.  The  old  water  wheel  was  even  more  trouble- 
some as  a  source  of  electrical  power  than  the  old  slide  valve 
steam  engine. 

The  customary  classification  of  water  wheels  for  many 
years  has  been  into  overshot,  undershot  and  breast  wheels, 
and  finally  turbines.  Various  modifications  of  all  these  have, 
of  course,  been  proposed  and  used.  Of  these  classes,  the  first 
three  may  be  passed  over  completely  as  having  no  importance 
whatever  in  electrical  matters,  save  in  certain  modifications  so 
different  from  the  original  wheelas  to  be  scarcely  recognizable. 
To  all  intents  and  purposes  they  are  never  used  for  the  pur- 
pose of  driving  dynamos,  although  occasionally  an  isolated 
instance  appears  on  a  very  small  scale. 

It  is  the  turbine  water  wheel  which  has  made  modern 
hydraulic  developments  possible,  and  more  particulary  elec- 
trical developments.  The  turbine  practically  dates  from  1827, 
when  Fourneyron  installed  the  first  examples  in  France, 
although  it  is  interesting  to  know  that  a  United  States  patent 
of  1804  shows  a  wheel  of  somewhat  similar  description,  never 
so  far  as  is  known  used.  The  modern  turbine  consists  of  two 
distinct  parts,  the  system  of  guide  blades  and  the  runner.  The 
runner  is  the  working  part  of  the  wheel,  and  consists  of  a 

324 


WATER    WHEELS.  325 

series  of  curved  buckets  so  shaped  as  to  receive  the  water  with 
as  little  shock  as  practicable,  and  to  reject  it  only  after  having 
utilized  substantially  all  of  its  energy.  These  buckets  are 
arranged  in  almost  every  imaginable  way  around  the  axis  of 
the  runner,  but  always  symmetrically. 

Sometimes  the  curvature  of  the  buckets  is  such  that  the 
water  after  having  passed  through  them  leaves  the  wheel 
parallel  to  its  axis;  sometimes  so  that  the  water  flows  inward 
and  is  discharged  at  the  center  of  the  runner;  sometimes  so 
that  it  passes  outward  and  is  discharged  at  the  periphery. 
More  often  the  buckets  have  a  double  curvature  so  that  the 
water  flows  along  the  axis  and  at  the  same  time  either  inwardly 
or  outwardly.  It  is  not  unusual  moreover  to  have  two  sets  of 
buckets  on  the  same  shaft  for  various  purposes.  The  growth 
of  the  art  of  turbine  building  has  made  any  classification  of 
turbines  depending  on  the  direction  of  the  flow  of  the  water 
uncertain,  as  in  nearly  every  American  turbine  this  flow 


FIG.  183.  FIG.  184. 

takes  place  in  more  than  one  general  direction,  usually  inward 
and  downward.  Aside  from  the  runner  the  essential  feature 
of  the  modern  turbine  is  the  set  of  guide  blades  which  sur- 
round the  runner,  and  which  are  so  curved  as  to  deliver  the 
water  fairly  to  the  buckets  in  such  direction  as  will  enable  it 
to  do  the  most  good.  Accordingly  these  blades  are  curved  in 
all  sorts  of  ways,  according  to  the  way  in  which  the  water  is 
intended  to  be  utilized. 

Fig.  183,  taken  from  Rankine,  shows  a  species  of  idealized 
turbine  which  discloses  the  principles  very  clearly.  In  this  fig- 
ure A  is  the  guide  blade  system  and  B  the  runner.  The  flow 
is  entirely  along  the  axis,  forming  the  so-called  parallel  flow 
turbine,  a  form  not  in  general  use  in  America. 

Fig.  184  shows  the  sort  of  curvature  which  is  given  to  the 


3.26  ELECTRIC   TRANSMISSION   O/<   POWER. 

guide  blades  and  to  the  buckets  of  the  runner.  The  axis  of 
this  or  any  other  kind  of  turbine  may  be  horizontal  or  vertical, 
as  convenience  dictates.  As  may  be  judged  from  the  illustra- 
tion, the  water  acts  on  the  runner  with  a  steady  pressure,  and 
the  buckets  of  the  runner  are  always  filled  with  the  water 
which  drives  them  forward.  Working  in  this  way  by  water 
pressure  due  to  the  weight  of  the  water  column,  it  is  not 
necessary  that  the  turbine  should  be  placed  at  the  extreme 
bottom  of  the  fall,  provided  an  air-tight  casing  is  continued 
below  the  runner  so  as  to  take  advantage  of  the  solid  water 
column  below  the  turbine.  Such  an  arrangement  is  called  a 
draft  tube,  and  may  be  of  any  length  up  to  the  full  column 
which  may  be  supported  by  atmospheric  pressure,  provided 
the  body  of  water  shall  be  continuous  so  that  there  shall  be  no 
loss  of  head  due  to  the  drop  of  the  water  from  the  wheel  to  the 
level  of  the  water  in  the  tail  race.  It  is  as  if  the  column  below 
were  pulling  and  the  column  above  pushing,  the  runner  being 
in  a  solid  stream  extending  from  the  highest  to  the  lowest 
level  of  water  used.  As  a  matter  of  practice  the  draft  tube  is 
generally  made  considerably  shorter  than  the  column  of  water 
which  might  be  supported  by  atmospheric  pressure,  generally 
less  than  20  feet,  depending  somewhat  on  the  size  of  the  wheel. 
With  longer  tubes  it  is  difficult  to  preserve  a  continuous  column, 
which  is  necessary  in  order  to  utilize  the  full  power  of  the 
water. 

Nearly  all  American  turbines  are  of  this  so-called  "  pressure  " 
type.  There  is,  however,  another  type  of  turbine  wheel  used 
somewhat  extensively  abroad,  and  occasionally  manufactured 
in  this  country,  which  without  any  very  great  change  in 
character  of  the  structure  operates  on  an  entirely  different 
principle.  There  are  present,  as  before,  guide  blades  deliver- 
ing the  water  to  the  buckets  of  the  runner,  but  the  spares  be- 
tween these  blades  are  so  shaped  and  contracted  as  to  deliver 
the  water  to  the  runner  as  a  powerful  jet.  The  energy  of 
water  pressure  is  converted  into  the  kinetic  energy  of  the 
spouting  jet,  and  the  buckets  of  the  runner  arc  not  filled  solidly 
and  smoothly  with  the  water,  but  serve  to  absorb  the  kinetic 
energy  of  the  jets,  and  discharge  the  water  below  at  a  very 
low  velocity.  Such  turbines  are  known  as  impulse  turbines, 
from  the  character  of  their  action.  In  the  pressure  turbines  the 


WATER    WHEELS. 


327 


full  water  pressure  acts  in  the  runner  and  in  the  space  between 
the  guides  and  the  runner.  In  the  pressure  turbine  each  space 
between  the  guide  blades  acts  so  as  to  form  a  water  jet,  which 
impinges  fairly  on  the  bucket  of  the  runner  without  causing  a 


GUIDE  VANES 


WHEEL 


FIG.   185. 

uniform  pressure  either  throughout  the  bilclcet  spaces  or  in  the 
space  between  runner  and  guides.  It  is  not  intended  that  the 
passages  of  the  wheel  should  be,  as  in  the  pressure  turbine, 


FIG.  186. 

entirely  filled  with  the  water,  nor  is  it  best  that  they  should  be. 
Fig.  185  gives  a  sectional  view  from  Unwin  showing  the 
arrangement  of  the  guide  blades  and  buckets  of  an  impulse 


328  ELECTRIC    TRANSMISSION  OF  POWER. 

turbine,  in  which  the  flow  is,  as  in  the  pressure  turbine  previ- 
ously shown,  in  general  along  the  axis  of  the  wheel.  An 
impulse  turbine  necessarily  loses  all  the  head  below  the  wheel 
and  cannot  be  used  with  a  draft  tube. 

Occasionally  an  attempt  is  made,  in  the  so-called  limit  tur- 
bines, so  to  design  the  guides  and  buckets  that  the  jets  may 
completely  fill  the  buckets,  which  are  adapted  exactly  to  the 
shape  of  the  issuing  stream.  In  such  case  the  turbine  works 
as  an  impulse  wheel  or  as  a  pressure  wheel,  according  as  the 
draft  tube  is  not  or  is  used. 


FIGS.   187  AND  188. 

A  modified  impulse  turbine,  largely  used  for  very  high  heads 
of  water,  is  found  in  the  Pelton  and  similar  wheels,  in  which 
the  impulse  principle  is  used  through  a  single  nozzle  acting  in 
succession  on  the  buckets  of  a  wheel  which  revolves  in  the 
same  plane  with  the  issuing  jet.  Such  a  Pelton  wheel  is  shown 
in  Fig.  186.  Occasionally  two  or  more  nozzles  are  used,  de- 
livering water  to  the  same  wheel.  Impulse  wheels  of  this  class 
are  exceedingly  simple  and  efficient,  and  work  admirably  on 
high  heads  of  water.  They  are  morever  very  flexible  in  the 
matter  of  obtaining  efficiently  various  speeds  of  rotation  from 
the  same  head  of  water,  as  the  wh@le  structure  is  so  simple 


WATER     WHEELS.  329 

and  cheap  that  it  can  be  modified  easily  to  suit  varying  condi- 
tions. 

It  is  obvious  that  the  operation  of  such  an  impulse  wheel  is 
simitar  to  that  of  a  true  impulse  turbine,  in  which  only  one, 
or  at  the  most  three  or  four  jets  from  the  guide  blades  are  util- 
ized. Most  of  the  hydraulic  work  done  in  this  country  is  ac- 
complished with  pressure  turbines,  which  are  worthy,  therefore, 
of  some  further  description.  A  small  but  important  por- 
tion is  accomplished  by  Pelton  and  other  impulse  wheels,  and 


FIG.  189. 

in  a  very  few  instances  the  impulse  turbine  proper  has  been 
used. 

There  are  manufactured  in  this  country  more  than  a  score 
of  varieties  of  pressure  turbines.  They  differ  widely  in  de- 
sign and  general  arrangement,  but  speaking  broadly  it  is  safe 
to  say  that  most  of  them  are  of  the  mixed  discharge  type,  in 
which  the  water  passes  away  from  the  buckets  of  the  runner 
inward  and  downward  with  reference  to  the  axis  of  the 
wheel.  It  would  be  impossible  to  describe  even  a  consider- 
able part  of  them  without  making  a  long  and  useless  cata- 


330 


ELECTRIC    TRANSMISSION   OF  POWER. 


logue.  The  essential  points  of  difference  are  generally  in  the 
construction  of  the  runner  and  in  the  mechanism  of  the  guide 
blades.  In  a  good  many  turbines  regulation  is  accomplished 
by  shifting  the  guide  blades  so  as  to  deliver  more  or  less  water 
to  the  runner.  A  few  types  will  serve  to  illustrate  the  general 
character  of  some  of  the  best-known  American  wheels,  Fig. 
187  shows  the  so-called  Samson  turbine  of  James  Leffel  & 
Co.,  and  Fig.  188,  the  ruwier  belonging  to  it.  This  wheel  is  of 


FIG.  190. 

the  class  which  regulates  by  shitting  the  guide  blades,  which 
are  balanced  and  connected  to  the  governor  by  the  rods  at  the 
top  of  the  casing  shown.  The  water  enters  the  guide  blades  in- 
wardly, and  the  runner  is  provided  as  shown  with  two  sets  of 
buckets;  the  upper  set  discharging  inwardly,  the  lower  and 
larger  set  downwardly.  The  action  of  the  wheel  is  almost 
equivalent  to  two  wheels  on  the  same  shaft,  the  intention  being 
to  secure  an  unusually  large  power  and  speed  from  a  given 
head  of  water  on  a  single  wheel  structure.  This  result  is, 
as  might  be  anticipated,  accomplished,  and  for  a  given  dia- 


WATER    WHEELS. 


331 


meter  the  Samson  turbine  has  a  speed  and  power  consider- 
ably greater  for  a  given  head  than  found  in  the  usual  standard 
single  wheels.  As  before  remarked,  however,  it  is  almost, 
mechanically  speaking,  equivalent  to  two  wheels  through  its 
peculiar  feature  of  double  discharge  through  independent 
buckets. 

Another  very  excellent  and  well-known  wheel  is  the  Victor 
turbine,  shown  in  Fig.  189.  In  this  wheel  the  gate  is  of  the  so- 
called  cylinder  type,  which  lengthens  or  shortens  the  apertures 


FIG.   191. 

admitting  water  to  the  guide  blades.  The  runner  of  this 
wheel  is  so  shaped  that  the  water  is  discharged  inwardly 
and  downwardly.  The  area  of  the  runner  blades  exposed 
to  the  full  water  pressure  is  notably  great.  The  cylinder 
form  of  gate  is  rather  a  favorite  with  American  wheel  man- 
ufacturers, and  is  intended  to  secure  a  somewhat  uniform 
efficiency  of  the  wheel,  both  at  full  and  part  load,  although 


332 


ELECTRIC    TRANSMISSION  OF  POWER. 


how  completely  it  does  this  is  a  matter  which,  of  course,  is  stilt1 
in  dispute.  The  wheel  shown,  however,  is  an  exceptionally 
good  and  efficient  one,  so  far  as  can  be  judged  from  general 
practice.  The  same  makers  also  manufacture  a  wheel  with 
shifting  guide  blades. 

Another  excellent  wheel  of  the  cylinder  gate  type,  the 
McCormick,  is  shown  in  Fig.  190.  The  runner  of  this  wheel  has 
its  main  discharge  downward.  It  has  a  rather  large  power  for 
its  diameter,  owing  to  the  proportion  of  the  runners,  and  is. 


FIG.  192. 

well  known  as  a  successful  wheel  considerably  used  in  driving 
electrical  machinery. 

These  turbines  are  typical  of  the  construction  and  arrange- 
ment used  by  first-class  American  manufacturers.  They  are 
all  arranged  for  either  horizontal  or  vertical  axes,  and  for 
purposes  of  driving  electrical  machinery  are  whenever  possible 
used  in  the  horizontal  form.  All  of  them,  particularly  the  two 
first  mentioned,  have  been  widely  used  for  electrical  purposes. 
They  are  all  practically  pure  pressure  turbines  and  are  installed 
usually  with  draft  tubes  of  appropriate  length.  They  are 
often,  too,  installed  in  pairs,  two  wheels  being  placed  on  the 
same  shaft,  fed  from  a  common  pipe  but  discharging  through 


WATER    WHEELS. 


333 


separate  draft  tubes.  The  arrangement  of  these  draft  tubes 
is  very  various,  as  they  can  be  placed  in  any  position  convenient 
for  the  particular  work  in  hand.  Fig.  191  shows  a  common 
arrangement  where  a  single  wheel  is  to  be  driven.  The  water 
enters  through  the  penstock,  passing  into  the  wheel  case, 
through  the  wheel,  .vhich  has,  as  is  generally  the  case  except 
with  very  low  heads,  a  horizontal  axis,  and  thence  passes  into 
the  tail  race  through  the  draft  tube,  shown  in  the  lower  part  of 
the  cut.  The  full  hea'd  in  the  particular  case  shown  is  43  feet, 
so  that  che  draft  tube  is  fairly  long.  Where  double  wheels  are 
employed,  there  is  no  longer  any  necessity  of  taking  up  the 
longitudinal  thrust  of  the  wheel  shaft,  and  an  arrangement 


FIG.  193. 

frequently  followed  is  shown  in  Fig.  192,  which  gives  a  very 
good  idea  of  the  general  arrangement  of  the  pair  of  horizontal 
turbines,  which  may  be  directly  coupled  to  the  load  or,  as  in 
the  case  just  mentioned,  drive  it  through  the  medium  of  belts. 
In  many  instances  it  is  found  cheaper  and  simpler  to  mount 
the  two  wheels  together  in  a  single  flume  or  wheelcase,  so  as 
to  discharge  into  the  same  draft  tube.  Fig.  193  shows  an 
arrangement  which  is  thoroughly  typical  of  this  practice, 
applied  in  this  case  to  a  low  head.  The  pair  of  wheels  are 
here  arranged  so  as  to  discharge  into  a  common  draft  tube 
between  them,  while  they  receive  their  water  from  the  timber 


334  ELECTRIC   TRANSMISSION  OF  POWER. 

penstock  in  which  they  are  inclosed.  Such  wooden  penstocks 
are  generally  very  much  cheaper  than  iron  ones  and  for  low 
heads  have  been  extensively  used. 

The  central  draft  tube  here  shown  need  not  go  vertically 
downwards,  but  may  take  any  direction  that  the  arrangement 
of  the  tailrace  requires.  Whether  the  draft  tube  is  single  or 
double  is  determined  mainly  by  convenience  in  arranging  the 
wheel  and  its  foundations,  and  the  tailrace.  The  use  of  a  pair 
of  turbines  coupled  together  is  not  only  important  in  avoiding 
end  thrust,  but  it  also  enables  a  fair  rotative  speed  to  be 
obtained  from  moderate  heads,  which  is  sometimes  very  im- 
portant in  driving  electrical  machinery. 

For  example,  suppose  one  desired  to  drive  a  500  KW  gener- 
ator by  turbines  from  a  25  ft.  head.  Allowing  a  little  margin 
for  overload  the  turbine  capacity  should  be  in  the  neighbor- 
hood of  750  HP.  Now  turning  to  a  wheel  table  applying,  for 
instance,  to  the  "Victor"  wheel,  one  finds  that  a  single  54" 
wheel  would  do  the  work,  but  at  the  inconveniently  low  speed  of 
128  r.p.m.  But  under  the  same  head  a  pair  of  39"  wheels 
would  give  a  little  larger  margin  of  power  at  180  r.p.m.,  and 
hence  would  probably  enable  one  to  get  his  dynamo  at  lower 
cost,  as  well  as  to  avoid  a  thrust  bearing.  Often  such  a  change 
of  plan  will  allow  the  use  of  a  standard  generator  where  a 
special  one  would  otherwise  be  necessary. 

Wherever  possible  it  is  highly  desirable  to  employ  these  hori- 
zontal wheels  for  electrical  purposes,  inasmuch  as  power  has,  in 
most  cases,  to  be  transferred  to  a  horizontal  axis,  and  the  use 
of  a  vertical  shaft  wheel  necessitates  some  complication  and 
loss  of  power  in  changing  the  direction  of  the  motion.  Occa- 
sionally a  vertical  shaft  wheel  is  used  for  electrical  purposes, 
driving  a  dynamo  having  a  vertical  armature  shaft.  This  prac- 
tice is  not  generally  to  be  recommended,  as  it  involves  special 
dynamos,  and  a  somewhat  troublesome  mechanical  problem 
in  supporting  the  weight  of  the  armature,  which  is  generally 
carried  by  hydraulic  pressure.  A  fine  example  of  this  arrange- 
ment is  to  be  found  in  the  great  Niagara  Falls  plant. 

The  use  of  pressure  turbines  for  driving  electrical  machinery 
is  exceedingly  convenient  on  low  or  moderate  heads,  say  up  to 
50  or  75  feet.  With  higher  heads  than  this  the  rotative  speed 
becomes  inconveniently  great;  for  example,  under  100  feet 


WA  TER    WHEELS. 


335 


head,  150  HP  can  be  obtained  from  a  wheel  a  little  more  than 
15  inches  in  diameter,  at  a  speed  of  more  than  1,000  revolu- 
tions per  minute.  At  200  feet  head,  the  power  for  the  same 
wheel  will  have  risen  to  about  400  HP  and  the  speed  to  nearly 
1,300.  This  is  a  rather  inconvenient  speed  for  so  large  a  power, 


FIG.  194. 

and  it  is  necessitated  by  the  fact  that  a  pressure  turbine  to 
work  under  its  best  conditions  as  to  efficiency,  must  run  at 
a  peripheral  speed  of  very  nearly  three-quarters  the  full 
velocity  of  water  due  to  the  head  in  question.  If,  therefore, 
turbines  are  used  for  high  heads,  either  the  dynamos  to 
which  they  are  coupled  must  be  of  decidedly  abnormal  design, 
or  the  dynamo  must  be  run  at  less  speed  than  the  wheels. 

The  former  horn  of  the  dilemma  was  taken  in  the  Niagara 
plant,  and  involved  some  very  embarrassing  mechanical  ques- 
tions in  the  construction  of  the  dynamos.  Where  belts  are 
permissible  the  other  practice  is  the  more  usual,  of  which  a 
good  example  is  found  in  the  large  lighting  plant  at  Spokane 
Falls,  Wash.,  where  the  wheels  are  belted  to  the  dynamos 
for  a  reduction  in  speed  instead  of  an  increase,  as  is  usually 
the  case. 


336  ELECTRIC    TRANSMISSION   OF  POWER. 

Impulse  wheels  are  regularly  made  at  present  by  only  one 
American  company,  the  Girard  Water  Wheel  Co.  of  San  Fran- 
cisco, Cal.  The  wheel  manufactured  by  them  is  one  with  a  well- 
known  foreign  reputation.  Its  general  arrangement  is  well 
shown  by  the  diagram,  Fig.  194.  The  Girard  impulse  turbine  is 
of  the  outward  flow  type,  a  form  rather  rare  in  pressure  turbines. 
The  water  enters  the  wheel  centrally  through  a  set  of  guide 
blades,  which  form  a  series  of  nozzles  from  which  the  water 
issues  with  its  full  spouting  velocity  and  impinges  on  the 
buckets  of  the  runner,  which  surrounds  the  guide  blades. 
The  discharge  is  virtually  radially  outward.  Regulation  is 
secured  by  a  governor  which  either  cuts  off  one  or  more  of  the 
nozzles  or  may  be  arranged  by  swinging  guide  blades  to  con- 
tract all  or  a  part  of  the  nozzles.  In  either  case,  th^re  is  no 
water  wasted,  and  the  wheel  works  efficiently  at  practically  all 
loads. 

Like  others  of  the  impulse  type,  the  peripheral  speed  of  the 
wheel  when  worked  under  its  best  conditions  for  efficiency  is 
very  nearly  one-half  the  spmiting  velocity  of  the  water  as  it 
issues  from  the  nozzle.  This  produces  for  a  wheel  of  given 
diameter  a  lower  speed  for  the  same  head  than  in  the  case  of 
pressure  turbines,  while  the  use  of  a  larger  number  of  nozzles 
working  simultaneously  on  the  runner  gives  a  higher  power  for 
the  same  diameter  than  in  the  case  of  the  Pelton  or  similar 
wheels,  which  use  a  few  nozzles  with  jets  applied  tangentially; 
hence  such  impulse  turbines  occupy  an  exceedingly  useful  place 
in  the  matter  of  speed,  aside  from  all  questions  of  efficiency. 

Under  moderately  high  heads,  from  100  up  to  300  or  400 
feet,  they  give  a  much  greater  power  for  a  given  rotative 
speed  than  impulse  wheels  employing  only  two  or  three 
nozzles.  On  the  other  hand  they  do  not  run  inconveniently 
fast,  as  is  the  case  with  pressure  turbines  under  such 
heads.  At  extremely  high  heads  they  give,  unless  operated 
with  only  one  or  a  few  nozzles,  so  great  power  as  to  be  incon- 
venient for  the  highspeed  attained,  so  far  at  least  as  the  oper- 
ation of  dynamos  is  concerned.  At  very  low  heads  there 
is  material  loss  from  the  fact  that  the  wheel  eannot  be  used 
with  the  draft  tube,  and  consequently  a  certain  amount  of  the 
head  must  be  sacrificed  to  secure  free  space  from  the  wheel  ta 
the  tail  water.  These  Girard  turbines  are  made  with  both. 


WATER    WHEELS.  337 

vertical  and  horizontal  axes,  and  are  applicable  to  electrical 
work  with  the  same  general  facility  which  applies  to  other 
types  of  wheel.  Their  strong  point  is  economical  and  effi- 
cient regulation  of  the  water  supply,  together  with  high  effi- 
ciency at  moderate  loads. 

The  Pelton  wheel,  already  shown  in  Fig.  186,  may  be  regarded 
as  an  impulse  turbine  having  a  single  nozzle,  and  that  applied 
tangentially.  These  wheels  have  proved  immensely  effective 
for  heads  from  several  hundred  up  to  a  couple  of  thousand  feet. 
Like  the  true  impulse  turbines,  the  peripheral  speed  should  be 
half  the  spouting  velocity  of  the  water,  hence  by  varying  the 
dimensions  of  the  wheel  a  wide  range  of  speed  can  be  obtained, 
which  is  exceedingly  convenient  in  power  transmission  work, 
permitting  direct  coupling  of  the  dynamos  under  all  sorts  of 
conditions.  They  are  not  infrequently  made  with  two  or  three 
nozzles,  which  give,  of  course,  correspondingly  greater  power 
for  the  same  speed.  At  heads  of  only  100  or  200  feet  these 
wheels  with  their  few  nozzles  give  an  inconveniently  low  rota- 
tive speed  for  the  power  developed,  and  are  at  their  best  in 
this  respect  between  300  and  1,000  feet.  The  Pelton  wheel 
is  usually  regulated  by  deflecting  the  nozzles  away  from  the 
buckets  of  the  wheel,  a  very  effective  but  most  inefficient 
method,  so  far  as  economy  of  water  is  concerned.  The  wheel 
has,  however,  under  favorable  conditions  a  very  high  efficiency, 
•certainly  as  high  as  can  be  reached  with  any  other  form  of 
hydraulic  prime  mover.  The  practical  results  given  by  this 
class  of  wheel,  both  of  Pelton  and  other  makes,  are  admirable 
under  circumstances  favorable  to  their  use. 

Another  wheel  of  this  class  is  the  Leffel  "  Cascade  "  water 
wheel.  Two  complete  rings  of  buckets  are  employed  for  this 
wheel,  instead  of  the  divided  buckets  of  the  Pelton  form, 
and  the  wheels  are  arranged  to  be  supplied  from  several  noz- 
zles, of  which  one  or  more  are  put  into  use  according  to  the 
necessities  of  regulation.  The  Cascade  wheel  therefore 
occupies  a  place,  as  it  were,  between  the  Pelton  wheel  and  the 
impulse  turbine,  resembling  the  former  in  the  arrangement  of 
its  multiple  jets,  and  the  latter  in  the  method  of  regulation  by 
cutting  off  completely  some  of  the  nozzles. 

From  the  foregoing  it  will  be  appreciated  that  each  of  the 
three  general  classes  of  wheels  described,  pressure  turbines, 


338  ELECTRIC    TRANSMISSION   OF  POWER. 

impulse  turbines  and  tangential  impulse  wheels,  has  a  sphere 
of  usefulness  in  which  it  can  hardly  be  approached  by  either 
of  the  others.  It  is  worth  while,  therefore,  to  examine  some- 
what in  detail  the  conditions  of  economy  under  various 
circumstances. 

The  pressure  turbine  has  its  best  field  under  relatively  low 
and  uniform  heads.  By  means  of  the  draft  tube  no  head  is 
lost,  as  is  the  case  with  that  portion  of  the  head  which  lies 
between  the  turbine  and  the  tail  water  in  the  use  of  impulse 
wheels  of  any  description.  Further,  the  pressure  turbine  under 
all  heads  gives  a  higher  relative  speed  than  the  impulse  wheels, 
whether  of  the  tangential  or  turbine  variety,  and  under  low 
heads  is  apt  to  be  of  less  bulk  and  cost  and  to  give  a  more  con- 
venient speed  for  electric  work;  hence  these  pressure  turbines 
have  been  more  extensively  used  than  any  other  variety  of 
water  wheels  in  the  enormous  hydraulic  developments  of  the 
last  quarter  of  a  century.  Furthermore,  the  pressure  turbine 
has,  under  favorable  conditions,  as  high  efficiency  as  any  known 
variety  of  water  wheel.  The  losses  of  energy  are  mainly  of 
four  kinds: 

1.  Friction  of  bearings,  usually  small. 

2.  Friction  and  £ddying  in  the  wheel  and  guide  passages. 

3.  Leakage,  and 

4.  Unutilized  energy  of  other  kinds,  largely  owing  to  imper- 
fect shaping  of  the  working  parts,  or  loss  of  head. 

With  the  best  construction  these  losses  aggregate  15  to 
20  per  cent.  Of  them  the  shaft  friction  is  the  smallest  and 
the  less  from  friction  and  eddies  in  the  wheel  the  largest, 
probably  fully  half  of  the  total  loss,  particularly  under  high 
heads.  This  efficiency  is  approximately  true  of  the  better 
class  of  turbines,  whether  of  the  pressure  or  impulse  variety. 
Under  low  and  uniform  heads  the  pressure  turbine  probably  is- 
capable  of  a  little  better  work  than  the  impulse  variety,  but  both 
suffer  if  the  head  varies.  The  curves,  Fig.  195,  show  efficiency 
tests  made  with  the  greatest  care  on  four  first-class  pressure  tur- 
bines at  the  Holyoke  testing  flume,  probably  the  best  equipped 
place  in  the  world  for  making  such  tests.  It  should  be  noted 
that  the  efficiency  of  all  the  wheels  shown  is  good;  over  80 
per  cent,  at  full  admission  of  water;  at  partial  admission 
the  efficiencies  vary  more  between  the  individual  wheels.  This 


WATER    WHEELS. 


339 


variation  is  largely  due  to  the  methods  of  regulating  'he  flow 
employed.     These  are  in  general  three: 

1.  Varying  the  number  of  guide  passages  in  use. 

2.  Varying  the  area  of  these  guide  passages  by  moving  the 
guide  blades. 

3.  Varying  the  admission  to  the  guide  passages  by  a  gate 
covering  the  entrance  to  all  of  them. 

The  first  method  is  particularly  bad,  as  the  buckets  are  at  one 
moment  exposed  to  full  water  pressure  and  then  come  opposite 
a  closed  passage,  setting  up  a  good  deal  of  unnecessary  shock 
and  eddying.  It  is  a  method  that  is  scarcely  ever  used  in  this 
country.  Between  the  other  two  it  is  not  so  easy  to  choose. 
Both  have  strong  advocates  among  wheel  makers;  some  com 


100 
85 
90 


o  80 

fc 
75 


70 


65 


60 


PROPORTIONAL  DISCHARGE 
FlG.  195. 

panics  building  both  types,  and  the  others  only  one  of  them. 
The  curves  shown  represent  both  these  methods  of  regulation. 
The  truth  probably  is  that  the  relative  efficiency  of  the  two 
depends  more  on  the  design  of  the  wheel  with  reference  to  its 
particular  form  of  regulation,  than  on  the  intrinsic  advantages 
of  either  form.  Turbines  are  generally  constructed  so  that 
the  point  of  maximum  efficiency  is  rather  below  the  maxi- 
mum output,  as  a  little  lee-way  is  desired  for  purposes  of  regu- 


34°  ELECTRIC  TRANSMISSION  OF  POWER. 

lation  under  varying  heads,  so  that  the  design  is  arranged  to 
give  the  best  efficiency  of  which  the  wheel  is  capable  at  a  point 
a  little  below  full  admission.  These  efficiency  curves  were 
taken  at  heads  of  from  15  to  18  feet  and  show  what  can  be  regu- 
larly accomplished  by  good  wheel  design.  They  are  neither 
phenomenally  high  nor  unusually  low.  Occasionally  efficiencies 
are  recorded  slightly  better  than  those  shown.  In  this  connec- 
tion it  is  desirable  to  state  in  the  way  of  warning,  that  there 
was  obtained  at  the  Holyoke  flume  some  years  ago  a  series 
of  tests  of  turbines  of  more  than  one  make,  which  showed 
enormously  high  efficiency,  afterward  traced  to  a  constant 
error  in  the  experiments.  As  the  fact  of  the  error  was 
not  so  generally  known  as  the  result  of  the  tests,  occasion- 
ally reports  are  heard  of  phenomenal  turbine  efficiencies  which 
are  given  in  entire  good  faith,  but  based  on  errors  of  experi- 
ment. It  is  only  fair  to  say  that  the  tests  now  made  at 
the  Holyoke  flume  are  worthy  of  entire  confidence. 

As  regards  impulse  turbines,  data  are  hard  to  obtain. 
Those  which  are  available  indicate,  however,  that  with  an  effi- 
ciency probably  a  little  less  at  full  load  than  that  of  pressure 
turbines  under  moderate  heads,  the  half  load  efficiency  is 
generally  considerably  higher.  This  is  owing  to  the  fact  that 
the  buckets  of  the  runner  work  entirely  independently  of  each 
other,  and  the  water  acts  in  precisely  the  same  way  on  each 
bucket  whether  it  is  received  from  all  the  nozzles  formed  by  the 
guide  blades,  or  from  a  part  of  them.  The  impulse  turbines  are 
generally  regulated  by  cutting  off  more  or  less  of  the  nozzles. 
The  shaping  of  the  surfaces  in  the  runner  and  guide  blades, 
and  the  smoothness  of  the  finish,  are  of  more  importance  in 
these  wheels  than  in  the  ordinary  pressure  turbines.  The 
impulse  turbines  are,  as  has  already  been  stated,  peculiarly 
adapted  in  point  of  speed  and  general  characteristics  for  use 
on  moderately  high  heads,  and  in  this  work  they  give  a  better 
average  efficiency  and  more  economical  use  of  water  than  any 
of  the  pressure  turbines.  For  low  heads  their  advantages  are 
far  less  marked,  and  the  pressure  turbines  are  generally 
preferred. 

The  tangential  impulse  wheels  are,  at  full  admission  of  water, 
of  an  efficiency  quite  equal  to  that  of  the  best  turbines.  At 
partial  admission  they  cannot  be  expected  to  give  the  same 


WATER    WHEELS.  341 

results  as  do  the  best  impulse  turbines,  inasmuch  as  they  regu- 
late either  by  deflecting  the  nozzle  away  from  the  buckets, 
and  hence  wasting  water,  or  by  more  or  less  throttling  of  one 
or  more  of  the  individual  nozzles;  hence  using  the  water  less 
effectively.  In  this  latter  method  of  regulation,  there  are  dis- 
advantages arising  from  the  small  number  of  nozzles,  whereby 
it  is  impracticable  to  obtain  a  close  regulation  of  speed  by 
simply  cutting  off  nozzles  as  HI  the  impulse  turbines.  For 
very  high  heads,  however,  the  tangential  wheels  are  prefer- 
able to  any  turbines,  as  they  give  a  better  relation  between 
power  and  speed,  so  far  as  driving  electrical  machinery  is  con- 
cerned, and  their  extreme  simplicity  is  favorable  to  good  con- 
tinuous working  under  the  enormous  strains  produced  by  the 
impact  of  water  at  great  spouting  velocities. 

To  summarize,  pressure  turbines  are  admirable  for  low  and 
uniform  heads,  particularly  where  the  load  is  steady.  The 
impulse  turbines  give  more  efficient  use  of  water  at  part  load, 
and  a  more  convenient  speed  on  moderately  high  heads.  The 
tangential  impulse  wheels  do  relatively  the  best  work  under 
very  high  heads,  and  where  water  does  not  have  to  be  rigidly 
economized.  Each  of  the  three  classes  has  decided  advantages 
over  the  others  in  particular  situations,  and  the  full  load  efficiency 
of  all  three  is  approximately  equal.  The  choice  of  either  one 
of  these  types  should  be  made  in  each  individual  case  in  accord- 
ance with  the  hydraulic  conditions  which  are  to  be  met.  The 
choice  between  particular  forms  of  each  type  is  largely  a  com- 
mercial matter,  in  which  price,  guarantees,  facility  of  getting 
at  the  makers  in  case  of  repairs,  standard  sizes  fitting  the  par- 
ticular case  in  hand  and  similar  considerations  are  likely  to 
determine  the  particular  make  employed,  rather  than  any  broad 
difference  in  construction  or  operation. 

The  success  of  a  power  transmission  plant  depends  quite  as 
much  on  careful  hydraulic  work  as  on  proper  electrical  instal- 
lation. The  two  should  go  hand  in  hand,  and  any  attempt, 
such  as  is  often  made,  to  contract  for  the  two  parts  of  the 
plant  independently  of  each  other,  or  to  engineer  them  inde- 
pendently, generally  results  in  a  combination  of  electrical  and 
hydraulic  machinery  that  is  far  from  being  the  best  possible 
under  the  conditions,  and  is  quite  likely  to  be  anything  but 
satisfactory. 


342  ELECTRIC    TRANSMISSION  OF  POWER. 

The  hydraulic  and  electrical  engineers  should  go  over 
the  arrangement  of  the  plant  together  with  a  view  to  adapting 
each  class  of  machinery  to  the  other  as  perfectly  as  possi- 
ble, in  order  to  get  a  symmetrical  whole.  Many  troublesome 
questions  have  to  be  encountered,  and  only  the  closest  study 
will  lead  to  perfectly  successful  results. 

One  of  the  commonest  and  most  serious  difficulties  met  with 
in  laying  out  an  electrical  and  hydraulic  plant  for  transmission 
work,  lies  in  the  variability  of  the  head  of  water.  There  are 
comparatively  few  streams  from  which  can  be  obtained  an  in- 
variable head  practically  independent  of  low  water  or  freshets. 
The  usual  condition  of  things  is  to  find  a  fairly  uniform  head 
for  nine  or  ten  months  in  the  year,  and  rather  wide  variations 
during  the  remainder  of  the  time.  It  is  not  at  all  uncommon  to 
meet  a  water  powerwhich,  even  when  very  skillfully  developed, 
will  still  entail  upon  the  user  a  variation  of  25  or  30  per  cent. 
in  the  available  head. 

At  the  time  of  high  water  this  appears  as  a  rise  of  water 
level  in  the  tail  race,  so  as  to  diminish  the  head  available  for  the 
wheels.  In  times  of  low  water,  the  head  might  be  normal,  but 
the  quantity  of  water  altogether  insufficient.  Any  variations 
of  this  kind  are  of  a  very  serious  character,  because  they  not 
only  vary  the  amount  of  power  which  is  available,  but  they 
change  the  speed  of  the  wheels  so  that  the  dynamos  no  longer 
will  operate  at  their  proper  speed  and  hence  will  change  in 
voltage,  and  if  alternating  apparatus  is  used,  in  frequency  also, 
which  is  even  more  serious.  For  example:  Under  24  feet 
head  one  of  the  well-known  standard  wheels  gives  nearly 
650  HP  at  100  revolutions  per  minute.  Under  16  feet  head 
the  same  wheel  would  give  only  352  HP  at  82  revolutions. 

The  lack  of  power  occurring  at  the  time  of  high  water 
is  serious.  The  change  of  speed,  although  not  great,  is 
very  annoying,  and  should  be  avoided  if  possible.  Changes 
much  greater  than  this  are  common  enough.  The  season  of 
reduced  head  is  generally  short,  not  over  a  couple  of  months, 
often  only  a  week  or  two,  and  this  renders  the  situation 
doubly  embarrassing,  because  during  a  large  part  of  the  year 
the  same  wheel  must  be  able  to  operate  economically.  The 
methods  taken  to  get  out  of  this  difficulty  of  varying  head  are 
various;  most  of  them  bad.  One  of  the  commonest  is  to 


WATER    WHEELS.  343 

arrange  the  wheels  to  operate  normally  at  partial  gate,  then 
on  the  low  heads  to  throw  the  gate  wide  open  and  obtain 
increased  power.  On  the  high  heads  the  wheel  is  throttled 
still  more.  Such  an  arrangement  works  the  dynamo  in  a 
fairly  efficient  fashion,  but  the  wheel,  as  a  rule,  quite 
inefficiently  a  large  portion  of  the  time,  as  may  be  seen  by 
reference  to  the  efficiency  curves  of  the  wheels  just  given. 
It  is  a  practice  similar  to  that  whiQh  one  would  find  in  work- 
ing an  engine  at  part  load.  For  moderate  variations  of  head 
not  exceeding  10  per  cent.,  the  loss  of  efficiency  is  not  so 
serious  as  to  bar  this  very  simple  plan,  but  under  conditions 
too  frequently  encountered,  these  variations  of  efficiency 
would  be  so  great  as  to  make  the  method  exceedingly 
undesirable. 

Hydraulic  plants  are  occasionally  operated  without  any 
reference  to  economy  of  water,  and  in  such  cases  the  practice 
of  operating  normally  at  part  gate  is  frequently  followed, 
but  it  must  be  remembered  that  as  water  powers  are  more 
and  more  developed,  economy  of  water  becomes  more  and 
more  necessary,  and  in  every  case  should  be  borne  in  mind 
even  if  it  is  not  rendered  necessary  by  conditions  actually 
existing.  In  thoroughly  developed  streams  it  is  generally 
important  to  waste  no  water. 

Another  method  of  overcoming  the  difficulties  due  to 
variations  of  head  is  the  installation  of  two  wheels  on  the 
same  shaft,  one  intended  to  give  normal  power  and  speed  at 
the  ordinary  head,  the  other  at  the  emergency  head.  This 
practice  is  carried  out  in  various  forms.  Sometimes  two 
wheels  may  be  mounted  on  the  same  horizontal  or  vertical 
axis,  and  one  of  them  is  disconnected  or  permitted  to  run 
idle  except  when  actually  needed.  Another  modification  of 
the  same  general  idea  is  the  use  of  a  duplex  wheel  with  the 
runner  and  guides  arranged  in  two  or  three  concentric  sets  of 
buckets,  which  can  be  used  singly  or  together  according  to 
the  head  which  is  available. 

A  fine  example  of  this  practice  is  found  in  the  great  power 
plant  at  Geneva,  to  which  reference  has  already  been  made, 
where  the  head  varies  from  5^  to  12  feet.  Here  the  turbines 
have  buckets  arranged  in  three  concentric  rings,  the  outermost 
being  used  at  the  highest  head  and  all  three  at  the  lowest  head. 


344  ELECTRIC   TRANSMISSION  OF  POWER. 

Under  the  latter  condition  the  average  radius  at  which  the 
water  acts  upon  the  wheel  is  diminished  and  the  speed  is 
therefore  increased,  while  the  greater  volume  of  water  keeps 
up  the  power.  The  various  combinations  possible  with  the 
rings  of  buckets  are  so  effective  in  keeping  the  speed  uniform 
that  the  extreme  variation  of  speed  under  the  maximum  varia- 
tion of  head  is  only  about  10  per  cent.  Such  a  triplex  turbine 
is  of  high  first  cost,  but  is  decidedly  economical  of  water  at 
normal  load.  Still  another  variation  of  the  double  turbine 
idea  consists  of  installing  two  turbines  for  each  unit  of  power, 
one  acting  directly,  the  second  through  the  medium  of  belts. 
The  direct-acting  turbine  is  intended  for  normal  load,  the 
belted  turbine  of  larger  dimensions  for  use  during  the  periods 
of  low  head. 

This  arrangement  is  used  in  the  large  power  transmission 
plant  at  Oregon  City,  Ore.  It  is  economical  of  water,  but  is 
mechanically  somewhat  complicated.  It  is  probably  on  the 
whole  less  desirable  than  the  installation  of  two  turbines  on 
the  same  shaft,  and  much  less  desirable  than  the  duplex  or 
triplex  arrangement  just  referred  to.  Where  two  turbines  are 
operated  on  the  same  shaft,  it  is  generally  possible  to  arrange 
the  turbine  designed  to  operate  on  the  lower  head  so  as  to  run 
at  a  disproportionately  high  velocity  with  some  loss  of  effi- 
ciency, and  so  to  hold  the  speed  fairly  uniform. 

Still  another  method  of  counteracting  the  variation  of 
head  is  applicable  only  where  the  power  is  transmitted  from 
the  turbine  by  gears  or  belts.  In  this  case  it  is  always 
possible  to  operate  the  machinery  under  the  reduced  head 
with  some  loss  of  output,  but  still  at  or  near  the  proper 
speed.  Whatever  way  out  of  the  difficulty  is  chosen,  it 
should  be  borne  in  mind  that  the  most  desirable,  on  the  whole, 
is  the  one  which  will  work  the  wheels  during  the  generally 
long  period  of  fairly  steady  head  at  their  best  efficiency.  If 
there  is  to  be  any  sacrifice  of  efficiency,  it  should  by  all  means 
be  for  as  short  a  time  as  possible,  and  therefore  should  be  at 
the  periods  of  extreme  low  head.  At  such  times  water  is 
generally  plenty,  while  at  the  higher  heads  economy  in  its 
use  is  more  necessary. 


WATER    WHEELS.  345 

REGULATION    OF    WATER    WHEELS. 

For  many  years  there  have  been  bad  water  wheel  governors 
and  worse  water  wheel  governors,  but  only  recently  have  there 
appeared  governors  which  may  be  classified  as  good  from  the 
standpoint  of  the  electrical  engineer.  It  has  been  necessary 
to  go  through  the  same  tedious  period  of  waiting  and  experi- 
mentation that  was  encountered  before  dynamo  builders  could 
find  engines  which  would  hold  their  speed  at  varying  loads. 
Until  the  advent  of  electrical  transmission  work,  water  wheels 
were  most  generally  employed  for  certain  classes  of  manufac- 
turing, such  as  textile  mills,  where  the  speed  must  be  quite 
uniform,  but  where  at  the  same  time  the  load  is  almost  uniform ; 
or,  on  the  other  hand,  for  saw  mills  and  the  like%  where  constant 
speed  is  of  no  particular  importance. 

The  action  of  water  wheel  governors  as  regards  the  way  in 
which  they  vary  the  supply  of  water  is  very  different;  some 
merely  act  to  open  or  close  the  head  gates;  others  to  work  a 
cylinder  gate  immediately  around  the  wheel,  and  still  others 
to  vary  the  area  of  the  guide  passages,  as  in  the  so-called 
register  gate  turbines. 

In  whatever  way  the  governing  action  takes  place,  its 
result  is  too  often  unsatisfactory,  due  to  the  great  difficulty 
that  has  to  be  encountered  in  the  great  inertia  of  the  water 
and  of  the  moving  parts  of  the  wheel.  Both  water  and  wheel 
are  sluggish  in  their  action,  and  as  a  result  some  time  elapses 
after  the  governor  has  produced  a  change  of  gate  before 
that  change  becomes  effective.  Meanwhile  the  speed  has 
fallen  or  risen  to  a  very  considerable  extent,  and  perhaps  in 
addition  the  load  has  again  changed  so  that  by  the  time  the 
speed  of  the  wheel  has  been  sensibly  affected  by  the  governor, 
the  direction  of  the  governing  action  may  be  exactly  opposite 
to  that  which  at  the  moment  is  desirable.  Even  if  this  is  not 
the  case  the  governing  is  usually  carried  too  far,  being  con- 
tinued up  to  the  time  at  which  the  wheel  is  affected  and  reacts 
on  the  governing  apparatus,  hence  another  motion  of  the 
governor  becomes  necessary  to  counteract  the  excess  of  dili- 
gence on  the  part  of  the  first  action.  In  other  words,  the 
governor  **  hunts,"  causing  a  slow  oscillation  of  the  speed 
about  the  desired  point,  an  oscillation  of  decreasing  amplitude 
if  the  new  load  on  the  wheel  be  steady. 


346  ELECTRIC   TRANSMISSION   OF  POWER. 

This  sluggishness  of  reaction  to  changes  indicated  by  the 
governor  is  the  most  formidable  obstacle  to  the  proper  control 
of  the  water  wheels.  To  overcome  it,  even  in  part,  it  is 
necessary  that  the  movement  of  the  gates  be  comparatively 
active,  if  the  changes  of  load  are  frequent,  and  this  entails  still 
further  difficulty  by  causing  severe  strains  on  the  mechanism 
and  the  gates,  particularly  if  the  water  is  led  to  the  turbine 
through  a  long  penstock.  In  the  latter  case  the  variations  in 
pressure  produced  by  rapid  governing  are  often  dangerous, 
and  have  to  be  counteracted  by  air  chambers,  stand  pipes  or 
the  like,  and  aside  from  all  this  there  is  a  still  further  difficulty 
in  the  considerable  weight  of  the  gates  and  the  pressure 
against  which  they  have  to  be  operated,  so  that  the  amount 
of' mechanical  power  controlled  by  the  governor  must  be  very 
considerable. 

A  very  large  variety  of  governors  have  been  designed  to 
meet  the  very  serious  difficulties  just  set  forth.  Most  of 
them  have  been  abject  failures,  and  those  that  may  be  really 
accounted  of  some  considerable  value  for  electrical  work  may 
be  counted  on  the  fingers  of  one  hand. 

Water  wheel  governors  may  be  roughly  divided  into  two 
classes.  First  come  those  regulators  in  which  the  wheel  itself 
supplies  power  to  the  gate-shifting  mechanism,  which  is  con- 
trolled by  a  fly  ball  governor  through  more  or  less  direct 
mechanical  means.  Second  comes  the  relay  class  of  governors, 
wherein  all  the  work  possible  is  taken  off  the  centrifugal 
governor,  and  its  function  is  reduced  to  throwing  into  action 
a  mechanism  for  moving  the  gates  which  may  be  quite  inde- 
pendent of  any  power  transmitted  from  the  wheel  to  the  gov- 
erning mechanism.  The  various  classes  of  hydraulic,  pneu- 
matic and  electric  governors  are  worked  in  this  way.  Their 
general  characteristic  is  that  their  sole  function  in  governing 
is  to  work  the  devices  which  control  the  secondary  mechanism, 
which  consists,  in  various  cases,  of  hydraulic  cylinders  oper- 
ating the  gates,  pneumatic  cylinders  serving  the  same  pur- 
pose, or  electric  motors  which  open  or  close  the  gates  by 
power  derived  from  the  machines  operated  by  the  turbines. 

A  vast  amount  of  ingenuity  has  been  spent  in  trying  to 
work  out  regulators  of  the  first  mentioned  form.  Almost 
every  possible  variety  of  mechanism  has  been  employed 


WATER    WHEELS.  347 

to  enable  the  governor  to  apply  the  necessary  power  to  the 
mecnanism  operating  the  gates.  The  general  form  of  most  of 
these  governors  is  as  follows:  Power  is  taken  from  the  wheel 
shaft  by  a  belt  to  the  governor  mechanism,  where  it  serves  at 
once  to  drive  the  governor  balls,  and  to  work  the  gates  when 
the  governor  connects  the  gate-controlling  gears  to  the  pulley 
which  supplies  the  power.  This  is  generally  done  by  friction 
cones  or  their  equivalent,  thrown  into  action  in  one  direction 
when  the  governor  balls  rise  and  in  the  other  direction  when 
they  fall. 

Sometimes  this  mechanism  is  varied  by  employing  a  pair 
of  oscillating  dogs,  one  or  the  other  of  which  is  thrown 
into  appropriate  gearing  by  the  governors.  There  are  many 
governors  of  this  kind  on  the  market,  and  where  the  load  is 
fairly  steady  and  no  particular  accuracy  of  regulation  is  neces- 
sary, they  have  given  good  satisfaction.  The  fault  with  all 
governors  of  this  sort  is  that  the  centrifugal  balls  either  lack 
sensitiveness  or  lack  power.  If  the  governor  works  at  all 
rapidly  in  moving  the  gates,  too  heavy  a  load  is  thrown  on 
the  governor  for  any  but  a  massive  mechanism,  and  the  cen- 
trifugal device  becomes  insensitive;  or,  on  the  other  hand,  if 
the  gates  are  worked  slowly,  the  governor  in  itself  is  sluggish 
and  ineffectual. 

In  most  cases  the  gates  are  made  to  move  quite  slowly. 
In  the  attempt  to  get  sensitiveness,  the  friction  wheels  or 
dogs  are  often  adjusted  so  closely  that  the  governor  is  in  a 
constant  slight  oscillatory  motion,  but  when  its  action  is 
really  needed,  as  in  the  case  of  a  sudden  change  of  load, 
response  generally  does  not  come  quickly  enough.  It  is  of 
course  possible  to  construct  a  mechanical  relay  which  would 
possess  both  power  and  sensitiveness,  but  nearly  all  the 
governors  made  on  this  principle  lack  one  or  the  other,  and 
sometimes  both. 

The  second  type  of  governor,  as  mentioned,  is  not  open  to  the 
objections  noted,  if  properly  designed,  inasmuch  as  it  is  a 
comparatively  easy  matter  to  make  a  balanced  hydraulic  or 
pneumatic  valve  which  can  be  worked  even  by  the  most  sensi- 
tive of  governors,  and  yet  can  apply  power  enough  to  move 
heavy  gates  as  rapidly  as  is  consistent  with  safety.  In  ad- 
dition, such  governors  can  be  made  to  work  with  a  rapidity 


ELECTRIC   TRANSMISSION  OF  POWER. 

depending  on  the  amount  of  change  in  speed,  so  that  it  a 
heavy  load  is  thrown  on  the  wheel,  the  relay  valve  would 
be  thrown  wide  open  and  conseouentlv  brine  a  great  and 
immediate  pressure  to  bear  upon  the  gates.  In  the  so-called 
electric  governors,  the  function  of  the  governor  balls  is  merely 
to  make  in  one  direction  or  the  other  the  electrical  con- 
nections to  a  reversible  motor  which  handles  the  gates. 
This  relay  class  of  governors  has  been  recently  worked  out 
with  considerable  care,  and  is  capable  of  giving  surprisingly 
close  regulation  even  under  widely  varying  loads,  results 
comparable  even  with  those  obtained  from  a  steam  engine 
governor. 

A  third  type  of  water  wheel  governor  is  independent  of 
any  centrifugal  device  and  operates  by  a  differential  speed 
mechanism,  so  that  wherever  the  speed  of  the  wheel  varies  from 
a  certain  fixed  speed  maintained  by  an  independent  motor, 
the  gates  are  opened  or  closed  as  occasion  demands.  The 
difficulty  here  is  to  get  a  constant  speed  which  will  not  be 
sensibly  altered  when  the  load  of  working  the  gates  is  thrown 
on  the  governor  mechanism.  Some  species  of  relay  device 
is  almost  necessary  to  the  successful  operation  of  a  differential 
governor,  but  with  such  an  adjunct  very  close  regulation 
can  be  and  is  obtained. 

Up  to  the  past  few  years  almost  all  hydraulic  governing  has 
been  by  mechanisms  of  the  first  class,  and  it  is  only  recently 
that  the  relay  idea  has  been  worked  out  carefully,  both  for 
centrifugal  and  differential  mechanisms,  so  as  to  obtain  any- 
thing like  satisfactory  results  for  electrical  work  where  close 
regulation  of  speed  over  a  wide  variation  of  load  is  very 
necessary. 

For  electrical  purposes,  several  rather  interesting  governing 
mechanisms  have  been  tried,  which  do  not  fall  into  any  of  these 
classes,  inasmuch  as  their  function  is  to  keep  the  load  con- 
stant and  prevent  variations  of  speed  instead  of  checking  these 
variations  after  they  have  been  set  up.  Such  governors  (load 
governors  they  may  properly  be  called)  operate  by  electric 
means,  throwing  into  circuit  a  heavy  rheostat  or  a  storage 
battery  when  the  electrical  load  falls  off,  and  cutting  theso 
devices  out  again  when  the  load  in  the  main  ( ircuit  increases. 
These  governors  have  in  several  instances  been  applied 


WATER    WHEELS.  349 

with  success  to  controlling  the  variable  loads  found  in  electric 
railway  stations  operated  by  water  wheels.  But  they  waste 
energy  in  a  very  objectionable  manner,  and  at  best  can  only 
be  regarded  as  bad  makeshifts,  out  of  the  question  when  there 
must  be  any  regard  for  economy  of  water,  and  only  to  be 
tolerated  in  the  lack  of  an  efficient  speed  regulator. 

Occasionally  electric  governors  operated  by  the  variations 
in  the  voltage  of  the  circuit  supplied  have  been  tried,  but 
these  are  open  to  two  serious  objections :  In  the  first  place 
they  do  not  hold  the  voltage  steady  for  the  same  reasons  that 
most  speed  regulators  do  not  hold  the  speed  steady.  Secondly, 
they  regulate  the  wrong  thing.  In  transmission  plants,  most 
of  which  are  and  will  be  operated  by  alternating  currents,  it 
is  important  that  the  cycles  be  kept  uniform.  If  the  voltage 


FIG.  196. 

is  kept  constant  by  varying  the  speed,  the  cycles  are  sub- 
ject to  enough  variation  to  be  very  annoying  in  the  operation 
of  motors.  Automatic  voltage  regulators  working  through  vari- 
ation of  the  field  excitation  of  the  generator  belong  in  a  differ- 
ent catagory  and  have  come  into  considerable  and  successful 
use. 

To  pass  from  the  general  to  the  special,  Fig.  196  shows  a 
typical  water  wheel  governor  of  the  first  class,  that  is,  of  the 
kind  operated  directly  by  the  wheel  through  a  system  of  dogs 
worked  by  a  fly  ball  governor.  There  is  here  no  attempt  at 
delicate  relay  work,  and  the  resulting  mechanism,  while  quite 
good  enough  for  rough-and-ready  work,  is  of  little  use 
for  any  case  where  a  variable  load  must  be  held  to  its 


35°  ELECTRIC  TRANSMISSION  OF  POWER. 

speed  with  even  a  fair  degree  of  accuracy.  The  cut  shows 
the  construction  well  enough  to  render  further  description 
superfluous.  Governors  like  these  were  practically  the  best 
available  for  many  years,  and  proved  to  be  cheap  and  durable, 
but  they  seldom  governed  much  more  than  to  keep  the  wheels 
from  racing  dangerously  when  the  load  was  thrown  off,  or 
from  slowing  down  permanently  when  it  came  on.  It  is  not 
too  much  to  say  that  they  never  should  be  used  in  connection 
with  an  electrical  station,  unless  combined  with  intelligent 
hand  regulation — which  at  a  pinch  is  not  to  be  despised. 

Of  the  indirect  acting  and  relay  governors  there  are  many 
species,  most  of  which  had  better  be  consigned  to  the  oblivion 
of  the  scrap  heap.  But  out  of  the  manifold  inventions  and 
experiments  good  has  come,  so  that  at  the  present  time  there 
are  a  few  delicate  relay  governors  capable  of  holding  the  wheel 
speed  constant  within  a  very  narrow  margin  indeed.  Others 
of  similar  excellence  will  probably  be  evolved,  but  just  now 
three,  the  Lombard,  Replogle,  and  the  Faesch-Piccard,  together 
with  one  or  two  electrical  governors,  give  decidedly  the  best 
results.  The  first  named  has  given  very  remarkable  results 
in  several  transmission  plants  in  which  it  has  been  employed — 
results  quite  comparable  with  those  obtained  from  a  well-gov- 
erned steam  engine.  The  second  has  given  excellent  results 
in  the  Oregon  City  transmission  and  elsewhere,  while  the' last 
has  been  adopted  for  the  great  transmission  at  Niagara  Falls 
and  has  done  its  work  well.  All  three  are  somewhat  compli- 
cated and  expensive,  but  they  actually  do  govern  with  con- 
siderable precision. 

The  Lombard  governor,  Fig.  197,  is  an  hydraulic  relay  in 
principle.  The  gate-actuating  mechanism  is  a  rack  gearing 
into  a  pinion,  and  driven  to  and  fro  by  the  piston  of  a  pressure 
cylinder.  The  working  fluid  is  thin  oil,  kept  under  a  pressure 
of  about  200  Ibs.  per  square  inch.  This  pressure  is  supplied 
by  a  pump  driven  by  the  pulley  shown  in  the  figure  and 
operating  to  keep  up  a  200  Ib.  air  pressure  in  the  pressure 
chamber  at  the  base  of  the  governor,  above  the  oil  that  par- 
tially fills  it.  This  chamber  is  divided  into  two  sections,  the 
one  holding  the  oil  under  pressure,  the  other  being  a  vacuum 
space  kept  at  reduced  pressure  by  the  pump  system. 

The  circulation  of  oil  is  from  the  pressure  chamber  through 


WATER    WHEELS.  35 * 

the  pipingsystem  and  valves  to  the  working  cylinder,  and  thence 
into  the  vacuum  chamber,  whence  it  is  pumped  back  into  the 
pressure  chamber  again.  The  governor  proper  consists  of 
a  sensitive  pair  of  fly  balls  operating  a  balanced  piston  valve 
in  the  path  of  the  pressure  oil.  A  motion  of  -fa  of  an  inch  at 
the  valve  is  sufficient  to  put  the  piston  into  full  action  and 
open  or  close  the  gates.  Sensitive  as  this  mechanism  is,  it 
would  not  govern  properly  without  the  addition  of  an  ingenious 
device,  peculiar  to  this  governor,  to  take  account  of  the  inertia 
of  the  system.  The  weakest  point  of  all  such  governing 
mechanisms  has  been  their  helplessness  in  the  matter  of  inertia. 
If  a  sensitive  governor  of  the  first  class  be  set  to  regulate  a 


FIG.  197. 

wheel  we  encounter  the  following  unpleasant  dilemma:  If  the 
mechanism  moves  the  gates  quite  slowly,  it  will  not  be  able  to 
follow  the  changes  of  load.  If  it  moves  them  rapidly  the 
governing  overruns  on  account  of  the  inertia  of  the  whole 
wheel  system,  so  that  the  apparatus  "  hunts,"  perhaps  the 
worst  vice  a  governor  can  have  when  dynamos  are  to  be  gov- 
erned. Hence  most  governors  have  either  been  unable  to 
follow  a  quickly  varying  load  at  all,  or  they  have  made  matters 
worse  by  hunting. 

In  the  Lombard  governor  special   means  are  provided  to 


WATER    WHEELS. 


353 


obviate  hunting.  The  bell-crank  lever  seen  in  the  background 
of  Fig.  197  is  actuated  by  the  same  movement  that  works  the 
wheel  gates,  and  moves  the  governor  valve  independently  of 
the  fly  balls.  Its  office  is  promptly  to  close  the  valve  far  enough 
ahead  of  the  termination  of  the  regular  gate  movement  to 
compensate  for  inertia.  For  example,  if  the  speed  falls  and 


FIG.  ig8A. 

the  fly  balls  operate  to  open  the  gate  wider,  the  lever  in  ques- 
tion closes  the  governor  valve  before  the  fly  balls  are  quite 
back  to  speed,  so  that  instead  of  overrunning  and  hunting,  the 
governing  is  practically  dead  beat. 

The  result  obtained  with  this  governor  is  well  seen  in  Fig.  198. 
This  diagram  is  taken  from  a  plant  operating  an  electric  street 
railway — perhaps  the  worst  possible  load  in  point  of  irregu- 


354  ELECTRIC  TRANSMISSION  OF  POWER. 

larity.  The  diagram  shows  a  maximum  variation  of  2.1  per 
cent,  from  normal  speed,  lasting  less  than  one  minute,  under 
extreme  variation  of  load.  These  results  are  entirely  authen- 
tic, the  readings  having  been  taken  jointly  by  the  represent- 
atives of  the  governor  company  and  the  local  company. 
Speed  was  taken  by  direct  reading  tachometer  and  load  from 
the  station  instruments.  A  still  more  instructive  exhibit  of 
the  performance  of  this  remarkable  governor  is  Fig.  ipSA,  which 
is  the  regular  working  record  of  an  electric  railway  station  for 
twenty-four  hours.  The  road  was  one  where  the  variations  of 
load  are  more  than  usually  severe,  and  the  day  was  in  mid- 
winter with  more  or  less  troubles  on  the  line,  the  card  showing 
no  less  than  four  short-circuits  of  considerable  severity.  But 
in  spite  of  all  disturbing  influences  the  voltage  was  held 
remarkably  well.  Even  the  heavy  short-circuits  produced 
only  a  momentary  effect,  and,  as  the  card  shows  both  the 
variations  in  voltage  due  to  speed  and  to  compounding,  the 
efficacy  of  the  governing  is  obvious.  Few  stations  of  similar 
output  driven  by  steam  would  show  a  better  result. 

A  gate  gauge  is  generally  attached  to  the  bed  plate  of  the 
Lombard  governor,  so  that  the  excursions  of  the  piston  plainly 
show  the  exact  extent  of  gate  opening.  The  mechanism  of 
the  governor  is  decidedly  complicated,  but  it  is  extremely 
well  made  and  fitted,  so  that  it  seldom  gets  out  of  order.  It 
permits  readily  of  all  sorts  of  adjustment  with  respect  to  the 
speed,  but  for  power  transmission  work  one  needs  constant 
speed  only,  except  when  varying  speed  temporarily  in  syn- 
chronizing a  generator.  The  invariable  rule  therefore  should 
be  to  adjust  the  governor  carefully  for  the  ex'act  speed  required, 
and  thereafter  to  LET  ITS  ADJUSTMENTS  ALONE  as  long  as  it 
continues  to  hold  that  speed.  In  power  transmission  work  and 
in  railway  plants  this  governor  is  at  present  used  probably 
more  than  all  others  combined,  but  neither  it  nor  any  other 
governor  can  give  first-class  results  unless  the  hydraulic 
equipment  generally  is  designed  with  some  degree  of  intelli- 
gence. 

The  Faesch  &  Piccard  governor  has  taken  several  forms, 
the  idea  of  a  sensitive  relay  mechanism  being  carried  through 
all  of  them.  An  hydraulic  relay  has  been  successfully  em- 
ployed abroad.  In  this  the  function  of  the  fly  balls  is  reduced. 


356 


ELECTRIC   TRANSMISSION  OF  POWER. 


to  moving  a  balanced  valve  controlling  hydraulic  power 
derived  from  the  natural  head,  or  from  a  pressure  cylinder. 
There  is  no  mechanical  provision  against  hunting,  but  the 
speed  of  governing  is  adjusted  as  nearly  as  possible  to  the  re- 
quirements of  the  load,  and  the  results  are  generally  good.  In 
the  great  Niagara  plant  the  governor  is  situated  on  the  floor 


FIG.  200. 

of  the  power  house,  nearly  140  feet  above  the  wheel.  It  is  a 
very  sensitive  mechanical  relay,  in  which  the  motion  of  a  pair 
of  fast  running  fly  balls  puts  into  operation  through  a  system  of 
oscillating  dogs  a  brake-tightening  mechanism  which  in  its  turn 
permits  power  to  be  transferred  from  pulleys  driven  from  the 
turbine  shaft  through  a  pair  of  dynamometer  gears  to  the  system 
of  gearing  that  works  the  balanced  gate  at  the  end  of  the  lever 


358 


ELECTRIC   TRANSMISSION   OF  POWER. 


system  140  feet  below  the  governor.  This  governor  was  guar- 
anteed to  hold  the  speed  constant  within  2  per  cent,  under  ordi- 
nary changes  of  load,  and  to  limit  the  speed  variation  to  4  per 
cent,  for  a  sudden  change  of  25  per  cent,  in  the  load.  Fig.  199 
give  a  good  notion  of  the  principles  of  this  apparatus,  which 
is  fairly  satisfactory.  The  Replogle  governor  is  an  electro- 
mechanical relay  shown  in  Fig.  200,  which  exhibits  its  general 
arrangement  very  well.  The  work  done  by  the  fly  balls  is 


FIG.  202. 

very  trifling  and  the  mechanism  is  both  sensitive  and  powerful. 
Fig.  201  shows  its  performance  in  governing  a  railway  load 
under  conditions  of  unusual  severity.  As  in  Fig.  198,  20  min- 
utes of  operation  are  plotted  and  the  maximum  variation  from 
105  revolutions  per  minute,  the  normal  speed,  is  less  than  TO 
revolutions,  and  that  variation  lasted  less  than  20  seconds  and 
was  due  to  the  opening  of  the  circuit-breaker.  Such  work  is 
quite  good  enough  to  meet  all  ordinary  conditions. 

To  a  very  different  type  of  mechanism  belongs  the  differen- 
tial governor  shown  in  Fig.  202.  It  has  been  applied  widely  to 
the  governing  of  Pelton  impulse  wheels,  with  very  excellent 
results.  The  principle  involved  is  very  simple.  Two  bevel 


WATER    WHEELS.  359 

gears,  each  carrying  on  its  shaft  a  pulley,  are  connected  by  a 
pair  of  bevel  gears  on  a  crosswise  shaft,  forming  a  species  of 
dynamometer  gearing.  Normally  the  main  gears  are  driven 
in  opposite  directions,  the  one  at  a  constant  speed  by  a  special 
source  of  power,  the  other  from  the  shaft  to  be  governed. 
So  long  as  the  speeds  of  these  wheels  are  exactly  equal  and 
opposite,  the  transverse  shaft  remains  stationary  in  space  and 
the  gate  moving  mechanism  attached  to  it  is  at  rest.  When, 
however,  the  working  shaft  changes  speed  tinder  the  influence 
of  a  change  in  load,  the  transverse  shaft  necessarily  moves  in 
one  direction  or  the  other  and  keeps  on  moving  until  the 
working  shaft  gets  back  to  speed. 

In  practice  the  main  difficulty  is  to  hold  the  constant  speed 
necessary  for  one  of  the  bevel  gears,  and  the  governor  works 
admirably  or  badly  as  this  constancy  is  or  is  not  maintained. 
A  heavy  fly  wheel  on  the  constant  speed  side  is  desirable,  and 
its  motive  power  should  be  quite  independent  of  the  main 
drive.  Perhaps  the  best  result  is  obtained  by  using  a  second, 
small,  differential  governor  to  hold  the  speed  uniform  at  the 
main  governor.  With  the  high  heads  and  balanced  deflecting 
nozzles  usual  in  Pelton  wheel  practice,  this  form  of  governor 
is  very  sensitive  and  does  not  hunt  noticeably,  owing  to  the 
small  inertia  of  the  moving  parts.  It  gives  good  regulation 
under  all  conditions  except  extreme  variations  of  load,  where 
the  wheel  is  loaded  beyond  the  power  of  the  jet  to  enforce 
prompt  recovery  of  speed,  and  is  singularly  well  suited  to  the 
conditions  under  which  Pelton  wheels  are  generally  used. 

The  greatest  difficulty  in  hydraulic  governing  is  that  of 
hydraulic  inertia.  Water  moves  sluggishly  through  long  and 
level  pipes,  and  its  velocity  does  not  change  promptly  enough 
for  good  governing"  unless  the  waterways  are  planned  with 
that  in  view.  If  a  wheel  is  at  the  end  of  a  long  and  gently 
sloping  penstock  it  takes  a  certain  definite  amount  of  time  to 
get  that  water  column  under  way  or  checked  in  response  to 
the  movement  of  the  gates.  And  the  longer  this  time  con- 
stant of  the  water  column  the  more  difficult  it  is  to  get  accu- 
rate governing,  however  good  the  governing  mechanism  may 
be.  For  by  the  time  the  water  gets  fairly  into  action  the  load 
conditions  may  have  changed  and  the  governor  may  be  again 
actively  at  work  trying  to  readjust  the  speed. 


360  ELECTRIC    TRANSMISSION  OF  POWER, 

In  order  to  get  accurate  governing  it  is  absolutely  necessary 
to  keep  the  time  constant  of  the  water  ways  as  small  as  pos- 
sible. To  accomplish  this  the  regulating  gate  should  obviously 
be  right  at  the  wheel  and  the  penstock  should  be  as  short  and 
as  nearly  vertical  as  possible.  The  most  favorable  condition 
for  governing  is  when  the  wheel  is  practically  in  an  open 
flume.  If  steel  penstocks  are  used  they  should  pitch  as 
sharply  down  upon  the  wheels  as  conditions  permit,  some- 
thing after  the  manner  of  Fig.  192.  If  long  head  pipes  must 
toe  used  governing  will  become  difficult,  although  much  help  can 
be  obtained  from  an  open  vertical  standpipe  connected  with 
the  penstock  close  to  the  wheel.  The  contents  of  this  pipe 
serve  as  a  pressure  column  if  the  gate  is  suddenly  opened  and 
as  a  relief  valve  if  the  gate  suddenly  closes,  averting  the  some- 
times serious  pressure  due  to  the  violently  checked  stream. 
Plate  X  shows  such  a  standpipe  in  action  just  after  a  heavy 
electric  load  had  been  thrown  off.  The  water  normally  stands 
very  near  the  top  of  the  pipe,  which  begins  to  overflow  with  a 
slight  increase  of  the  hydraulic  pressure. 

Under  high  heads  such  a  standpipe  is  of  course  impracti- 
cable, and  although  some  forms  of  relief  valve  are  of  use,  the 
problem  of  governing  is  not  easy  until  one  comes  to  the  Pelton 
wheel  with  its  deflecting  nozzle. 

Not  all  water  wheels  are  governed  with  equal  ease.  If  the 
gates  are  properly  balanced  a  comparatively  small  amount  of 
power  will  manage  them  promptly  and  the  wheel  is  governed 
without  trouble.  But  there  are  some  wheels  on  the  market 
with  gates  under  so  much  unbalanced  pressure  that  proper 
governing  is  difficult  or  impossible.  There  is  no  excuse  for 
the  existence  of  such  wheels,  for  they  do  not  have  compensat- 
ing advantages,  and  they  should  be  shunned.  All  the  typical 
wheels  which  have  been  described  in  this  chapter  govern 
easily,  however,  as  do  many  others.  It  is  worth  while  to  re- 
member that  good  governing  is  absolutely  indispensable  for 
good  service,  and  although  one  finds  cases  in  which  the  load  is 
so  steady  that  the  wheels  can  almost  go  without  governing, 
such  are  rare  exceptions  to  the  general  rule. 


PLATE   X. 


CHAPTER  X. 

HYDRAULIC     DEVELOPMENT. 

So  much  electrical  transmission  work  depends  on  the  utiliza- 
tion of  water  powers  that  it  is  worth  while  briefly  to  consider 
the  subject  of  developing  natural  falls  for  such  use.  The 
subject  is  a  large  one,  quite  enough  to  fill  a  volume  by  itself, 
and  the  most  that  can  be  done  here  is  to  point  out  the  salient 
facts  and  put  the  reader  in  possession  of  such  information  as 
will  enable  him  to  avoid  serious  blunders  and  to  take  up  the 
subject  intelligently. 

Natural  water  powers  of  course  vary  enormously  in  their 
characteristics.  In  our  own  country,  where  water  power  is 
very  widely  distributed,  we  find  three  general  classes  of  powers, 
often  running  into  each  other  but  still  sufficiently  distinct  to 
cause  the  methods  of  developing  them  to  be  quite  well  defined. 

By  far  the  best  known  class  of  powers  are  those  derived 
from  the  swift  rivers  that  are  found  in  New  England  and 
other  regions  in  which  the  general  level  of  the  country  changes 
rather  rapidly.  They  flow  through  a  country  of  rocky  and 
hilly  character,  and  large  or  small,  are  still  swift,  powerful 
streams,  with  frequent  rapids  and  now  and  then  a  cascade. 
Such  rivers  are  generally  fed  to  no  small  extent  by  springs 
and  lakes  far  up  toward  the  mountains,  and  catch  in  addition 
the  aggregated  drainage  of  the  irregular  hill  country  through 
which  they  flow.  Types  of  this  class  are  the  Merrimac  and 
the  Androscoggin  among  the  New  England  rivers,  the  upper 
Hudson,  and  many  others.  Another  and  quite  different  class 
of  powers  are  those  derived  from  the  slow  streams  that  flow 
through  a  flat  or  rolling  alluvial  country — the  Mississippi 
valley  and  the  lowlands  of  the  Southern  Atlantic  States.  Al- 
though possessed  of  many  tributaries  that  spring  from  among 
the  mountains,  the  great  basins  which  they  drain  form  the 
main  reliance  of  rivers  of  this  kind — immense  areas  of  fertile 
country  the  aggregated  rainfall  of  which  supports  the  streams. 

361 


362  ELECTRIC   TRANSMISSION  OF  POWER. 

Finally,  there  are  many  fine  water  powers  that  come  from 
mountain  streams,  fed  from  little  mountain  springs,  from  the 
melting  of  the  winter  snows  and  the  drainage  of  heights  which 
the  snow  never  deserts,  and  from  the  rain  gathered  by  moun- 
tain gorges. 

These  mountain  rivers  often  furnish  magnificent  powers, 
easy  and  cheap  to  develop,  but  very  variable.  In  summer  the 
stream  may  dwindle  to  a  mere  brook,  while  in  spring,  from 
the  combined  effect  of  rains  and  melting  snow,  it  will  suddenly 
increase  even  many  thousand  fold,  becoming  a  tremendous 
torrent  that  no  works  built  by  man  can  withstand.  The 
available  heads  are  often  prodigious,  from  a  few  hundred  to 
more  than  a  thousand  feet,  and  the  volume  of  water  may  seem 
at  first  sight  absurdly  small,  but  when,  as  in  the  Fresno  (Cal.) 
plant  to  be  described  later,  each  cubic  foot  flowing  per  second 
means  140  mechanical  HP  delivered  by  the  wheels,  large 
volume  is  needless. 

Upland  rivers  like  those  common  in  New  England  seldom 
give  opportunity  for  securing  high  heads.  Most  of  the  powers 
developed  show  available  falls  ranging  from  20  to  40  feet. 
Unless  the  stream  has  considerable  volume,  such  low  heads  do 
not  yield  power  enough  to  serve  anything  but  trivial  purposes — 
only  two  or  three  HP  per  cubic  foot  per  second.  Upland 
rivers,  however,  furnish  the  great  bulk  of  the  water  power  now 
utilized,  for  they  furnish  fairly  steady  and  cheap  power  under 
favorable  conditions.  Although  subject  to  considerable, 
sometimes  formidable,  freshets  when  the  snow  is  melting  or 
during  heavy  rains,  they  are  generally  controllable  without 
serious  difficulty. 

Lowland  streams  seldom  offer  anything  better  than  very 
low  heads,  rarely  more  than  10  to  15  feet,  and  consequently 
demand  an  immense  flow  to  produce  any  considerable  power. 
They  are,  however,  as  a  class  rather  reliable.  The  size  and 
character  of  the  drainage  basin  makes  extremely  low  or 
extremely  high  water  rare,  and  only  to  be  caused  by  very 
great  extremes  in  the  rainfall.  Such  streams  furnish  a  vast 
number  of  very  useful  powers  of  moderate  size,  forming  a 
large  aggregate  but  seldom  giving  opportunity  for  any  striking 
feats  of  hydraulic  engineering,  at  least  in  our  own  country, 
where  fuel  is  generally  cheap. 


HYDRAULIC  DEVELOPMENT.  363 

In  taking  up  any  hydraulic  work  with  reference  to  electrical 
power  transmission,  or  any  other  purpose,  in  fact,  the  first 
necessary  steo  is  to  make  a  sort  of  reconnaissance,  to  ascertain 
the  general  topography  of  the  region,  the  available  head,  and 
the  probable  flow.  The  first  two  points  are  generally  easy  to 
determine  from  existing  surveys  or  by  a  brief  series  of  levels, 
the  last  named  requires  a  combination  of  educated  judgment 
and  careful  engineering.  The  facts  are  not  really  very  diffi- 
cult to  get  at,  but  guesswork  is  emphatically  out  of  order 
and  hearsay  evidence  even  more  worthless  than  usual.  The 
author  has  seen  more  than  one  mighty  torrent  dwindle  into  a 
trout  brook  when  looked  at  through  untinted  spectacles. 

The  only  way  to  find  out  how  much  flow  is  available  is  to 
measure  it  carefully,  if  it  has  not  already  been  measured  in  a 
thorough  and  trustworthy  manner — not  once  or  twice  or  a 
dozen  times,  but  weekly  or,  better,  daily  for  an  entire  season  at 
least;  the  more  thoroughly  the  better.  A  knowledge  of  the 
absolute  flow  at  one  particular  time  is  interesting,  but  of  little 
value  compared  with  a  knowledge  of  the  variations  of  flow 
from  month  to  month,  or  year  to  year. 

Such  a  series  of  measurements  tells  two  very  important 
things — first,  the  minimum  flow,  which  represents  the  max- 
imum power  available  continuously  without  artificial  storage 
of  water;  and  second,  the  aggregate  flow  during  any  specified 
period,  which  shows  the  possibilities  of  eking  out  the  water 
supply  by  storage. 

The  methods  of  measurement  are  comparatively  simple. 
For  small  streams  the  easiest  way  is  to  construct  a  weir  across 
the  stream  and  measure  the  flow  over  a  notch  of  known  dimen- 
sions in  this  weir.  Such  a  temporary  dam  should  be  tight  and 
firmly  set,  and  high  enough  to  back  up  the  water  into  a  quiet 
pool  free  from  noticeable  flow  except  close  to  the  edge  of  the 
weir.  There  should  be  sufficient  fall  below  the  bottom  of  the 
notch  in  the  weir  to  give  a  clear  and  free  fall  for  the  issuing 
water — say  two  or  three  times  the  depth  of  the  flow  over  the 
weir  itself. 

Fig.  203  shows  clearly  the  general  arrangement  of  a  measur- 
ing weir.  Here  A  shows  the  end  supports  of  the  weir,  here 
composed  of  a  single  plank,  while  B  is  the  lower  edge  of  the 
notch  through  which  the  water  flows.  This  edge  B,  as  well  as 


364 


ELECTRIC   TRANSMISSION  OF  POWER. 


the  sides  of  the  notch,  should  be  chamfered  away  to  a  rather 
sharp  edge  on  the  upstream  side,  which  must  be  vertical. 
Back  some  feet  from  the  weir  so  as  to  be  in  still  water  should 
be  set  firmly  a  post  E,  the  top  of  which  is  on  exactly  the  same 
level  as  the  bottom  of  the  weir  notch  B.  D  shows  this  level, 
while  the  line  C  shows  the  level  of  the  still  water.  The  quan- 
tities to  be  exactly  measured  are  the  length  of  the  notch  B 
and  the  height  from  the  level  of  the  edge  of  B  to  the  normal 


FIG.  203. 

level  surface  of  the  water  in  the  pool.  This  can  be  done 
generally  with  sufficient  accuracy  by  holding  or  fixing  a  scale 
on  the  top  of  the  post  E.  If  we  call  the  breadth  of  the  notch 
b,  and  this  height  h,  both  measured  in  feet,  the  flow  in  cubic 
feet  per  minute  is 

Q  =  40  c  b  h  \/2  g  h 

Here  g  is  32.2  and  c  is  the  "  coefficient  of  contraction,"  which 
defines  the  ratio  of  the  actual  minimum  area  of  the  flowing 
jet  to  the  nominal  area  b  h. 

This  coefficient  varies  slightly  with  the  width  of  the  notch 
as  compared  with  the  whole  width  of  the  weir  dam.  Calling 
this  w,  the  value  of  c  is  approximately 

b_ 
'"    -.        w 


HYDRAULIC  DEVELOPMENT. 


This  gives  c  =  .62  for  a  notch  half  the  width  of  the  weir  and 
c=  .67  for  the  full  width  of  the  weir.  For  notches  below  one- 
quarter  the  width  of  the  weir  the  values  of  c  become  somewhat 
uncertain,  and  as  a  rule  b  should  be  over  half  of  w.  Further, 
the  notch  should  not  be  so  wide  as  to  reduce  the  water  flowing 
over  it  to  a  very  thin  sheet.  It  is  best  to  arrange  the  notch  so 
that  the  depth  of  water  h  may  be  anywhere  a  tenth  to  a  half  of 
b.  For  purposes  of  approximation  weir  tables  are  sometimes 
convenient.  These  give  usually  the  flow  in  cubic  feet  per 
minute  corresponding  to  each  inch  in  width  £,  for  various, 
values  of  h.  Such  a  table,  condensed  from  one  used  by  one  of 
the  prominent  turbine  makers,  is  given  below.  Where  quite 
exact  measurement  is  required  the  constant  c  should  be 
determined  from  the  actual  dimensions  and  a  working  table 
deduced  from  it. 

TABLE  FOR  WEIRS. 


INCHES  AND  FRACTIONS 
DEPTH  ON  WEIR. 

O 

X 

y2 

X 

0.40 

o  56 

o  74 

O.Q7 

2 

1.14 

i  16 

I   ^Q 

i  84 

a 

2  OQ 

2  16 

2  64 

2  Q*l 

4  

1.22 

a   ca 

3.85 

4  17 

5" 

A   ei 

4  8*. 

c  2<; 

*  <;6 

6                                         .    . 

5  Q2 

6  ^o 

6  68 

7  O7 

7    . 

7  46 

7  87 

8  28 

8  70 

8  

9-12 

9.55 

9.QQ 

10.43 

10.88 

11.34 

II.  80 

12  27 


10  . 

12  75 

11  23 

11  72 

14  21 

ii  

14.71 

15  21 

ic.72 

16.24 

12  

16.76 

17.28 

17.82 

18.35 

11 

18  80 

IQ  44 

20  oo 

2O  56 

14  • 

21    12 

21  68 

22  26 

22  83. 

1C.  . 

2*3.42 

24.OI 

24.60 

25.IQ 

16  

25.80 

26.41 

27.O2 

27.63 

17    . 

28  26 

28  88 

2Q  e.i 

3O.I4 

18  . 

00.78 

•31    A1 

12  O7 

32.73 

Cubic  feet  per  minute  per  inch  of  width. 

West  of  the  Rocky  Mountains  a  special  system  of  measuring 
water  by  "  miner's  inches"  has  come  into  very  extensive  use. 
It  originated  in  the  artificial  distribution  of  water  for  mining 
and  irrigating  purposes,  and  has  since  extended  to  a  conven- 
tional measurement  for  streams.  The  miner's  inch  is  a  unit 


366 


ELECTRIC    TRANSMISSION  OF  POWER. 


of  constant  flow,  and  varies  somewhat  from  State  to  State,  its 
amount  being  often  regulated  by  statute  in  various  States:  It 
is  the  flow  through  an  aperture  i  inch  square  under  a  speci- 
fied head,  frequently  6  inches.  The  method  of  measurement 
is  shown  in  £  ig.  204.  The  water  is  led  into  a  measuring  box 
closed  at  the  end  except  for  an  aperture  controlled  by  a  slide. 
The  end  board  is  i£  inch  thick,  and  the  aperture  is  2  inches 


FIG.  204. 

wide,  its  bottom  is  2  inches  above  the  bottom  of  the  box,  and  its 
centre  6  inches  below  the  level  of  the  water.  Each  inch  of 
length  of  the  aperture  then  represents  2  miner's  inches. 
Under  these  conditions  the  flow  is  1.55  cubic  feet  per  minute  for 
each  miner's  inch.  Under  a  4^  inch  effective  head,  which  is 
extensively  used  in  Southern  California  and  the  adjacent 
regions,  the  miner's  inch  is  about  1.2  cubic  feet  (9  gallons) 
per  minute. 

For  streams  too  large  to  be  readily  measured  by  the  means 
already  described,  a  method  of  approximation  is  applied  as 
follows: 

Select  a  place  where  the  bed  of  the  stream  is  fairly  regular 
and  take  a  set  of  soundings  at  equal  intervals,  a,  b,  c,  d.  Fig. 
205,  perpendicular  to  the  direction  of  flow,  using  a  staff  rather 
than  a  sounding  line,  as  it  can  more  easily  be  kept  perpendicu- 


HYDRAULIC  DEVELOPMENT. 


367 


lar.  Ascertain  thus  the  area  of  flow.  Then  establish  two  lines 
across  the  stream  say  100  feet  apart  and  nearly  equidistant 
from  the  line  of  soundings.  Then  throw  floats  into  the 
stream  near  the  centre  and  time  their  passage  across  the  two 
reference  lines.  This  establishes  the  velocity  of  the  flow 
across  the  measured  cross  section.  As  the  water  at  the  bottom 
and  sides  of  the  channel  is  somewhat  retarded,  the  average 
velocity  is  generally  assumed  to  be  80  per  cent,  of  that  meas- 
ured as  above  in  the  middle  of  the  stream. 

The   more   complete   the   data   on   variations  of   flow,   the 
better.     The  most  important  point  to  be  fixed  is  the  flow  at 


c    d 


^J^ 

^^ 

FIG.  205. 


extreme  low  water,  both  in  ordinary  seasons  and  seasons  of 
unusual  drought.  Except  on  very  well-known  streams  pre- 
vious data  on  this  point  are  generally  not  available.  The 
flow  should  therefore  be  measured  carefully  through  the  usual 
period  of  low  water  during  at  least  one  season.  From  the 
minimum  flow  thus  obtained  there  are  various  ways  of  judging 
the  minimum  flow  in  a  very  dry  year.  Sometimes  certain 
riparian  marks  are  known  to  have  been  uncovered  in  some 
particular  year,  and  the  relative  flow  can  be  computed  from  the 
difference  thus  established.  Again,  the  records  of  a  series  of 
years  may  be  obtained  from  a  neighboring  stream  of  similar 
character,  and  the  ratio  between  ordinary  and  extraordinary 
minima  assumed  to  be  the  same  for  both.  This  assumption 
must  be  made  cautiously,  for  neighboring  streams  often  are 
fed  from  sources  of  very  different  stability. 

Failing  in  these  more  direct  methods,  recourse  may  be 
taken  to  rainfall  observations.  For  this  purpose  the  rainfall 
in  the  basin  of  the  stream  should  be  measured  during  the  con- 
tinuance of  the  observations  on  flow.  By  noting  the  effect  of 


368  ELECTRIC   TRANSMISSION  OF  POWER. 

known  rainfall  on  the  flow  of  the  stream,  one  can  make  a 
fairly  close  estimate  of  the  flow  in  a  very  dry  year  in  which 
the  rainfall  is  known  by  months,  or  for  an  assumed  minimum 
rainfall.  In  a  similar  way  can  be  ascertained  the  probable 
high  water  mark,  record  of  which  is  often  left  by  debris  on 
the  banks. 

In  a  fairly  well-known  country  the  conditions  of  flow  can  be 
approximated  by  reference  to  rainfall  alone.  The  area  drained 
by  the  stream  down  to  the  point  of  utilization  can  be  closely 
estimated.  If  rainfall  observations  in  this  district  are  avail- 
able, or  can  be  closely  estimated  from  the  results  at  neighbor- 
ing stations,  one  may  proceed  as  follows:  The  total  water 
falling  into  the  basin  is  2,323,200  cubic  feet  per  square  mile 
for  each  inch  of  rainfall.  Only  a  portion  of  this  finds  its  way 
into  the  streams,  most  of  it  being  taken  up  by  seepage,  evapo- 
ration, and  so  forth.  The  proportion  reaching  the  streams 
varies  greatly,  but  is  usually  from  .3  to  .6  of  the  whole.  If  this 
proportion  is  known  from  observations  on  closely  similar 
basins  and  streams  the  total  yearly  flow  can  be  approximated, 
and  if  the  distribution  of  flow  on  a  similar  stream  is  known, 
one  can  make  a  tolerable  estimate  of  the  amount  and  condi- 
tions of  flow  in  the  stream  under  investigation. 

This  process  is  far  from  exact,  sinte  the  proportion  of  the 
total  water  which  is  found  in  the  streams  varies  greatly  from 
place  to  place,  and  with  the  total  rainfall  in  any  given  week  or 
day.  The  sources  of  loss  do  not  increase  with  the  total  pre- 
cipitation, and  the  only  really  safe  guide  is  regular  observa- 
tion of  the  rainfall  and  the  flow  during  the  same  period. 

A  good  idea  of  the  uncertainties  of  hydraulic  power  can  be 
gathered  from  the  recorded  facts  as  regards  the  Merrimac, 
one  of  the  most  completely  and  carefully  utilized  American 
streams,  which  has  been  under  close  observation  for  half  a 
century.  The  area  of  its  watershed  above  Lowell,  Mass.,  is 
4,093  square  miles  and  the  mean  annual  rainfall  of  the  region 
is  about  42  inches.  The  observations  of  many  years  indicate 
that  the  maximum,  minimum,  and  mean  flows  are  on  approxi- 
mately the  following  basis: 

(Spring),          .         .     Maximum,  90  cu.  ft.  per  min.  per  sq.  m. 
(June),    .         . '".'    .     Mean,          55       "  "  "       " 

(Aug.,    Sept.),         .     Minimum,  30       "  "  "      " 


HYDRAULIC  DEVELOPMENT.  369 

The  annual  rainfall,  if  it  all  could  be  reckoned  as  in  the 
stream  and  uniformly  distributed,  would  amount  to  very 
nearly  180  cubic  feet  per  minute  per  square  mile  of  watershed. 
In  fact,  this  flow  is  reached  or  passed  only  on  occasional 
days  of  heavy  freshets  during  the  spring  rains,  when  the  snow 
is  melting  rapidly.  The  normal  maximum  flow  is  just  50  per 
cent,  of  the  conventional  average,  while  the  real  average  falls 
to  about  30  per  cent,  and  the  minimum  to  less  than  17  per 
cent.  Of  late  years  this  minimum  has  sometimes  been  still 
smaller,,  barely  15  per  cent,  instead  of  17,  a  state  of  things 
attributed  to  the  destruction  of  the  forests  on  the  upper 
watershed.  In  a  heavily  wooded  country  the  rainfall  is  long 
retained  and  finds  its  way  to  the  streams  slowly  and  gradually. 
When  the  forests  are  cut  off  the  water  runs  quickly  to  the 
streams,  and  the  result  is  heavy  seasons  of  freshets  when  the 
snow  is  melting  —  all  the  more  rapidly  because  of  lack  of  forest 
shade  —  and  extreme  low  water  during  the  dry  months.  In  a 
bare  country  the  variations  of  flow  are  often  prodigious, 
and  without  storage  one  can  safely  reckon  only  upon  the 
minimum  flow  of  the  dryest  year.  As  the  denudation  of 
forests  goes  on  hydraulic  development  will  steadily  grow 
more  expensive. 

In  some  streams,  generally  in  hot  climates,  no  small  part 
of  the  flow  is  during  the  dry  season  in  the  strata  underlying 
the  apparent  bed  of  the  stream,  and  can  be  in  part,  at  least, 
captured  by  carrying  down  the  foundations  of  the  permanent 
works. 

When  the  flow  has  been  ascertained  the  available  HP  is 
easily  computed.  The  practicable  head  can  be  easily  deter- 
mined by  a  little  leveling.  If  H  is  this  head  in  feet  and  Q 
the  flow  in  cubic  feet  per  minute  then  the  theoretical  HP  of 
the  stream  is 

HP=   6^HQ 

33,000 

The  mechanical  HP  obtained  by  utilizing  the  stream  in  water 
wheels  is  this  total  amount  multiplied  by  the  efficiency  of 
the  wheels,  usually  between  .75  and  .85.  At  80  per  cent. 
efficiency  the  preceding  formula  reduces  to 


650 


37°  ELECTRIC    TRANSMISSION  OF  POWER. 

which  gives  the  available  mechanical  horsepower  directly,  in 
many  streams  the  available  head  is  limited  by  the  permissible 
overflow  of  the  banks  as  determined  by  the  rights  of  other 
owners,  or  by  danger  of  backing  up  the  stream  to  the  detri- 
ment of  powers  higher  up.  These  conditions  must  be  deter- 
mined by  a  careful  survey. 

Before  taking  up  seriously  the  development  of  a  water  power 
it  is  advisable  to  enter  into  an  examination  of  the  legal  status 
of  the  matter,  which  is  sometimes  involved.  The  general 
principle  of  property  in  streams  is  that  the  water  belongs  in 
common  to  the  riparian  owners,  and  cannot  be  employed  by 
one  to  the  detriment  of  another.  But  each  State  has  a  set  of 
statutes  of  its  own  governing  the  use  of  water  for  power  and 
other  purposes,  often  of  a  very  complicated  character,  involved 
with  special  charters  to  storage  and  irrigation  companies  and 
other  ancient  rights,  so  that  the  real  rights  of  the  purchaser  of 
a  water  privilege  are  often  limited  in  curious  and  troublesome 
ways,  especially  when  the  stream  has  been  long  utilized, 
elsewhere. 

Generally  the  riparian  owners  have  full  rights  to  the  nat- 
ural flow  of  the  stream,  which  is  often  by  no  means  easy  to 
determine.  The  laws  of  various  States  regarding  the  matter 
of  flowage  vary  widely,  and  altogether  the  intending  purchaser 
will  find  it  desirable  to  investigate  carefully  not  only  the  title 
to  the  property,  but  the  limitations  of  the  rights  which  he 
would  acquire. 

In  streams  of  small  volume  carried  through  pipe  lines  the 
effective  head  is  diminished  by  friction  in  the  pipes.  This 
loss  has  already  been  discussed  in  Chapter  II. 

It  often  happens  that  there  is  so  great  a  difference  between 
the  normal  flow  of  the  stream  during  most  of  the  year  and  its 
minimum  flow  during  a  few  weeks  as  to  make  it  highly  desira- 
ble to  store  water  by  impounding  it,  so  as  to  help  out  the 
sometimes  scanty  natural  supply.  With  mountain  streams 
under  high  head  this  is  frequently  quite  easy.  Even  when  it 
is  impracticable  to  impound  enough  to  help  out  during  the 
whole  low  water  period  it  is  sometimes  very  useful  to  impound 
enough  to  last  for  a  day  or  two  in  case  of  necessary  repairs. 

A  certain  reservoir  capacity  is  quite  necessary  so  as  to  per- 
mit the  storage  of  water  at  times  of  light  load  for  utilization 


HYDRAULIC  DEVELOPMENT.  371 

at  times  of  heavy  load.  This  process  is  carried  out  on  a  vast 
scale  on  the  New  England  rivers,  where  the  water,  used  during 
the  day  in  textile  manufacturing,  is  stored  in  the  ponds  at 
night  as  far  as  possible.  While  electric  transmission  plants  do 
not  offer  the  same  facilities  for  storage,  since  they  generally 
run  day  and  night,  the  application  of  the  same  process  would 
often  greatly  increase  their  working  capacity  and  greatly  lower 
the  fixed  charges  per  hydraulic  HP.  Such  storage  is  espe- 
cially valuable  in  cases  where  the  water  supply  is  limited,  as  it 
often  is  in  plants  working  under  high  heads.  Every  cubic  foot 
of  water  is  then  valuable  and  should  be  saved  whenever  possi- 
ble. Regulation  by  deflecting  nozzles,  which  is  very  generally 
employed  in  this  class  of  plants,  is  particularly  objection- 
able on  the  score  of  economy,  and  ought  to  be  replaced  by 
some  more  efficient  method. 

As  an  example  of  what  can  be  done  with  storage  under  high 
heads,  it  happens  that  at  650  feet  effective  head  one  mechani- 
cal HP  requires  almost  exactly  one  cubic  foot  of  water  per 
minute  at  80  per  cent,  wheel  efficiency.  For  a  500  HP  plant, 
then,  the  water  required  is  30,000  cubic  feet  per  hour. 

One  can  store  43,560  cubic  feet  per  acre  per  foot  of  depth, 
so  that  a  single  acre  10  feet  deep  would  store  water  enough  to 
operate  the  plant  at  full  load  for  14^  hours,  or  under  ordinary 
conditions  of  load  for  a  full  day.  If  the  flow  in  the  stream 
were  only  15,000  cubic  feet  per  hour  in  time  of  drought,  the 
acre  would  yield  two  days  supply  and  15  acres  would  carry  the 
plant  for  a  month.  Such  storage  is  common  enough  in  irriga- 
tion work,  and  is  capable  of  enormously  increasing  the  work- 
ing capacity  of  a  transmission  plant,  even  at  a  head  much  less 
than  that  mentioned. 

With  only  100  feet  available  head  it  is  comparatively  easy  to 
impound  water  enough  to  assist  very  materially  in  tiding  over 
times  of  heavy  load  and  in  increasing  the  available  capacity. 
A  survey  with  storage  capacity  in  view  should  be  made  when- 
ever storage  is  possible,  and  the  approximate  cost  of  storage 
determined.  A  little  calculation  will  show  in  how  far  storage 
will  pay. 

In  general  the  utilization  of  a  water  power  consists  in 
leading  the  whole  or  a  part  of  a  stream  into  an  artificial 
channel,  conducting  it  in  this  channel  to  a  convenient  point  of 


37 2  ELECTRIC   TRANSMISSION  OF  POWER. 

utilization,  and  then  dropping  it  back  through  the  water  wheels 
into  the  channel  again,  usually  via  a  tail  race  of  greater  or  less 
length. 

Except  where  there  is  a  very  rapid  natural  fall  a  sub- 
stantial dam  is  necessary,  which  backs  up  the  water  into  a 
pond,  usually  gaining  thus  a  certain  amount  of  head,  whence 
the  water  is  led  in  an  open  canal  to  some  favorable  spot  from 
which  it  can  be  dropped  back  into  the  channel  at  a  lower  level. 
The  canal  may  vary  in  length  from  a  few  rods  to  several  miles, 


FIG.  206. 

according  to  the  topography  of  the  country.  The  tail  race  lead- 
ing the  water  from  the  wheels  back  to  the  stream  is  short,  except 
in  rare  instances  like  the  great  Niagara  plant.  In  this  case, 
shown  somewhat  roughly  in  Fig.  206,  the  usual  construction 
was  reversed.  To  obtain  ample  clear  space  for  manufacturing 
sites  and  the  like,  the  water  was  utilized  by  constructing  above 
the  cataract  an  artificial  fall  at  the  bottom  of  which  the  wheels 
were  placed.  From  the  bottom  of  this  huge  shaft,  cut  178  feet 
deep  into  the  solid  rock,  the  water  is  taken  back  into  the 
river  through  a  tunnel  7,000  feet  long,  which  constitutes  the 
tail  race.  The  water  is  taken  directly  from  the  river  into 
the  canal  without  even  a  deflecting  dam. 

In  the  case  of  mountain  streams  having  a  very  rapid  fall 
the  dam  is  often  quite  insignificant,  serving  merely  to  back  up 
the  water  into  a  pool  from  which  it  may  be  conveniently  drawn, 
or  even  to  deflect  a  portion  of  the  water  for  the  same  purpose. 
In  such  cases  the  water  is  usually  carried  in  an  iron  or  steel 
pipe,  following  any  convenient  grade  to  the  bottom  of  the  fall 
chosen,  at  which  point  its  full  pressure  becomes  available. 


HYDRAULIC  DEVELOPMENT. 


373 


In  ordinary  practice  at  moderate  heads  the  volume  of  water 
has  to  be  so  considerable  for  any  large  power  as  to  make  a 
long  canal  very  expensive.  Further,  it  usually  happens  that 
the  topography  of  the  country  is  such  as  to  make  it  very  diffi- 
cult to  gain  much  head  by  extending  the  canal.  Thus  the 
points  chosen  for  power  development  must  be  those  where 
there  is  a  rather  rapid  descent  for  a  short  distance — falls  or 
considerable  rapids.  Then  a  dam  of  moderate  height  gives  a 
fair  head  by  simply  carrying  the  canal  to  a  point  where  the 
water  can  be  readily  returned  to  tlfe  stream  below  the  natural 
fall.  The  more  considerable  this  fall  the  less  need  for  an 
elaborate  dam,  which  may  become  simply  a  means  of  regulat- 
ing the  flow  of  water  without  noticeably  raising  the  head. 

A  fine  example  of  this  sort  of  practice  is  shown  in  Fig.  207, 
which  shows  a  plan  of  the  hydraulic  development  of  the  falls 
of  the  Willamette  River  at  Oregon  City,  Ore.  The  river  at 
this  point  gives  an  estimated  available  HP  of  50,000  under  40 


FIG    207. 

feet  head.  The  stream  plunges  downward  over  a  precipitous 
slope  of  rough  basalt,  and  the  low  dam  which  follows  the  some- 
what irregular  shape  of  the  natural  fall  is  hardly  more  than  an 
artificial  crest  to  guide  the  water  toward  the  canal  on  the  west 
bank  of  the  stream.  This  canal  has  recently  been  widened, 
and  both  constructions  are  shown  in  the  figure.  The  fine  three- 
phase  transmission  plant  of  the  Portland  General  Electric  Com- 
pany now  faces  on  the  new  canal  wall  near  the  section  G.  At 
the  end  of  the  canal  downstream  ^  series  of  locks  lead  down  to 
the  lower  river,  making  the  falls  passable  for  river  craft.  Only 
a  small  part  of  the  available  power  is  as  yet  used. 

Almost  every  river  presents  peculiarities  of  its  own  to  the 


374 


ELECTRIC  TRANSMISSION  OF  PO  WER. 


hydraulic  engineer.  Generally  the  dam  is  a  far  more  promi- 
nent part  of  the  work  than  at  Oregon  City,  and  adds  very 
materially  to  the  head.  Choosing  a  proper  site  for  the  dam, 
and  erecting  a  suitable  structure,  requires  the  best  skill  of  the 
hydraulic  engineer.  Bearing  in  mind  that  the  function  of  a 
dam  is  to  merely  retain  and  back  up  the  flowing  water,  it  is 
evident  that  it  may  be  composed  of  a  vast  variety  of  materials 
put  together  in  all  sorts  of  ways.  Stone,  logs,  steel,  all  come 
into  play  combined  with  each  other  and  with  earth. 

The  character  of  the  river  bed  which  furnishes  the  founda- 
tion is  a  very  important  factor  in  determining  the  material  and 
shape  of  the  dam  used.  When  the  bed  is  of  rock  or  that  hard 
packed  rubble  that  is  almost  as  solid,  a  well-built  stone  dam  is 
the  best,  as  it  is  also  the  costliest  construction.  For  such 
work  the  way  is  cleared  by  a  coffer  dam  and  the  masonry  is 
laid,  if  possible,  directly  upon  the  bedrock.  When  the  bottom 


FIG.  208. 

is  hard  pan  a  deep  foundation  for  the  masonry  is  almost  as 
good  as  the  ledge  itself,  while  on  a  gravel  bottom  sheet  piling 
is  sometimes  driven  and  the  stone  work  built  around  it.  The 
ground  plan  is  very  frequently  convex  upstream,  giving  the 
effect  of  an  arch  in  resisting  the  pressure  of  the  water.  Fig, 
208  shows  a  section  of  a  typical  masonry  dam,  built  over  sheet 
piling  in  heavy  gravel.  This  particular  dam  is  22'  6"  high 
and  nearly  300  yards  long.  The  coping  is  of  solid  granite 
slabs  a  foot  thick.  Below  the  dam  lies  the  usual  apron  of 
timber  and  concrete,  with  timber  sills  anchored  into  the  dam 
itself.  The  flooring  of  the  apron,  of  12"  X  12"  timbers  laid 
side  by  side,  is  bolted  to  the  foundation  timbers  laid  in  the 
concrete.  The  purpose  of  this  apron,  as  of  such  structures  in 


//  YDRA  ULIC  DE  VELOPMENT. 


375 


general,  is  to  prevent  undermining  of  the  dam  by  the  eddies 
below  the  fall. 

A  still  finer  example  of  the  masonry  dam  is  shown  in  Fig. 
209 — the  great  dam  of  the  Folsom  Water  Power  Company 
across  the  American  River  at  Folsom,  Cal.  It  is  built  of 
hewn  granite  quarried  on  the  spot,  and  is  founded  on  the  same 
ledge  from  which  the  material  was  taken.  The  abutments 
likewise  are  built  into  the  same  ledge.  On  the  crest  of  the 


TOP  OF  BULKHEAD 


|   TOP  OF  SHUTTER  AND 

WING  DAMS 

TOP  OF  DAM  IN  SHUTTER 
OPENINQ,SHUTTER  RAISED 

j    BY  5  HYDRAU.UC  RAMS 


SECTION   OF   DAM 
J  THRUST  — IQTI  TONS.        \ 
J  STABILITY— 7079  TONS.  | 

Containing  37,000  cu.  yards  of  masonry. 


FIG.  209. 

dam  proper  is  a  huge  shutter  or  flash  board,  185  feet  long, 
capable  of  being  swung  upward  into  place  by  hydraulic  power. 
When  thus  raised  it  gives  an  added  storage  capacity  of  over 
13,000,000  cubic  yards  of  water  in  the  basin  above.  This  dam 
furnishes  power  for  the  great  Folsom-Sacramento  transmission, 
and  it  ranks  as  one  of  the  finest  examples  of  hydraulic  engi- 
neering in  existence.  Including  the  abutments  it  is  470  feet 
long,  and  the  crest  of  the  abutments  towers  nearly  100  feet 
above  the  foundation  stones.  Its  magnificent  solidity  is  not 
extravagance,  for  the  American  River  carries  during  the  rainy 
season  an  enormous  volume  of  water,  filling  the  channel  far 


376  ELECTRIC  TRANSMISSION  OF  POWER. 

over  the  crest  of  the  dam  when  at  its  maximum  flow.     There 
are  few  streams  where  greater  strains  would  be  met. 

While  these  masonry  dams  are  splendidly  strong  and  endur- 
ing, they  are  also  very  expensive,  and  hence  unless  actually 
demanded  for  some  great  permanent  work  are  less  used  than 
cheaper  forms  of  construction.  In  many  situations  these  are 
not  only  cheaper  in  first  cost,  but  even  including  deprecia- 
tion. There  are  many  forms  of  timber  dam  which  have  given 
good  service  for  many  years  at  comparatively  small  expense. 
Of  such  dams,  timber  cribs  ballasted  with  stone  are  probably 
under  average  conditions  the  best  substitute  for  solid  masonry. 
These  crib  dams  when  well  built  of  good  materials  are  very 
durable  and  need  few  and  infrequent  repairs.  Some  such 
dams,  replaced  after  twenty-five  or  thirty  years  in  the  course  oi 
changing  the  general  hydraulic  conditions,  have  shown  timbers 
as  solid  as  the  day  they  were  put  down,  and  capable  of  many 
years'  further  service. 

A  fine  example  of  such  construction  is  the  dam  of  the  Con- 
cord (N.  H. )  Land  &  Water  Power  Company  at  Sewall's 
Falls  on  the  Merrimac.  A  section  of  this  structure  is  shown 
in  Fig.  210.  The  foundation  is  in  the  main  gravel,  in  which  the 
dam  is  made  secure  by  sheet  piling  and  stone  ballast.  The 
structure  is  essentially  a  very  solid  timber  crib  with  a  very  long 
apron.  The  total  head  is  23  feet,  of  which  more  than  half  is 
due  to  the  dam,  as  shown  in  the  levels.  The  apron  is  armored 
with  five-sixteenths  inch  steel  plate,  the  better  to  withstand  the 
bombardment  of  stray  logs  to  which  it  is  sometimes  subjected. 
The  abutments  are  of  granite.  It  has  proved  very  serviceable, 
having  successfully  withstood  several  tremendous  freshets  with 
no  damage  save  some  undermining  of  one  of  the  abutments, 
which  has  been  repaired  with  crib  work.  Considering  the 
character  of  the  river  bed,  this  dam  is  probably  as  reliable  as 
one  of  masonry,  and  its  cost  was  little  over  half  that  of  a 
masonry  dam. 

For  small  streams  these  ballasted  timber  dams  are  admirable, 
and  little  more  is  needed  in  most  cases. 

The  canals  leading  the  water  to  the  wheels  are  of  construc- 
tion as  varied  as  the  dams,  depending  largely  on  the  nature  of 
the  ground.  Sometimes  they  are  merely  earthwork,  oftener 
they  are  lined  with  timber,  concrete,  or  masonry.  Canal  con- 


HYDRAULIC  DEVELOPMENT. 


377 


struction  is  a  matter  to  be  decided  on  its  merits  by  the 
hydraulic  engineer,  and  very  little  general  advice  can  be 
given.  For  low  heads  wooden  pipes  made  of  staves  like  a  barrel 


378 


ELECTRIC   TRANSMISSION  OF  POWER. 


and  hooped  with  iron  every  3  or  4  feet  are  sometimes  used. 
In  many  situations  this  construction  is  cheaper  than  steel  pipe 
and  answers  admirably.  Such  wooden  pipes  are  considerably 
employed  in  the  West,  the  material  being  generally  redwood, 
and  have  proved  remarkably  durable,  some  having  been  in  use 
for  more  than  twenty  years.  Open  timber  flumes  are  also 
widely  used. 

For  very  high  heads4  canals  and  flumes  are  almost  univer- 


sally replaced  by  iron  or  steel  riveted  pipe  taken  by  the  nearest 
route  to  the  wheels  below.  This  practice  has  been  general 
on  the  Pacific  coast  and  has  given  admirable  results.  The 
pipes  are  asphalted  inside  and  out  to  prevent  corrosion,  and 
some  pipe  lines  have  been  in  service  for  a  quarter  of  a  cen- 
tury without  marked  deterioration.  Large  pipes  and  those  for 
very  heavy  pressures  are  usually  made  of  mild  steel.  The 
pipes  are  customarily  made  in  sections  for  shipment,  from 
20  to  30  feet  long,  and  the  slip  joints  are  riveted  or  packed  on 
the  ground.  For  transportation  over  very  rough  country  and 
for  very  large  pipes,  the  sections  may  be  no  more  than  2  or 


,3800ft. 


3600ft. 


Si 


0  600  ft.  1000  ft. 

FIG.  212. 

3  feet  long.  The  joints  are  then  asphalted  on  the  ground. 
Fig.  211  shows  several  of  these  short  sections  joined  together, 
exhibiting  the  nature  of  the  riveting  and  the  terminal  slip 
joint. 

In  running  such  a  pipe  line  it  is  usually  taken  in  as  straight 
a  course  as  possible,  and  is  laid  over,  on,  or  under  the  ground  as 
occasion  requires,  usually  on  the  surface,  conforming  to  its  gen- 
eral contour.  In  long  lines  the  upper  end  is  usually  larger  and 


H  YDRA  UL  1C  DE  VEL  OP  ME  N  T. 


379 


thinner  than  the  lower,  which  has  to  withstand  the  heavy 
pressure.  Fig.  212,  which  is  a  profile  to  scale  of  the  pipe  line 
of  the  noted  San  Antonio  Canon  plant  in  southern  California, 
gives  an  excellent  idea  of  good  modern  practice  in  this  sort  of 
work.  There  is  here  a  total  fall  of  about  400  feet  in  a  distance 
of  2,000  feet.  The  main  pipe  is  30"  in  diameter,  and  the  steel 
is  of  the  gauges  indicated  on  the  various  sections.  At  the 
•crests  of  two  undulations,  air  valves  are  placed  to  ensure  a 


Showing  method  of  anchoring  pipe  on  ft  steep 
Kr&dewith  examples  ol  lead  and  slip  joints , 


FIG.  213. 

solid  and  continuous  column  of  water  in  the  pipe.  The  last 
540  feet  of  pipe  is  reduced  to  24"  and  the  guage  of  steel  is 
somewhat  heavier.  The  total  length  of  the  pipe  line  is  2,370 
feet.  To  protect  the  pipe  against  great  changes  of  tempera- 
ture it  was  loosely  covered  with  earth,  rock,  and  brush  when- 
ever possible.  At  two  sharp  declivities  the  pipe  was  anchored 
to  the  rock. 

The  general  method  of  anchoring  on  a  steep  incline  is  shown 
In  Fig.  213.  In  this  case  the  slip-joint  is  simply  calked,  and 
where  consecutive  sections  are  at  an  angle,  a  short  sleeve  is 
fitted  over  the  joint  and  lead  is  run  in  as  shown  in  the  cut. 
Often  a  packed  slip  joint  is  used  very  freely,  thereby  gaining 
in  flexibility,  and  riveted  joints  may  be  only  used  occasionally. 
The  line  is  generally  started  from  the  lower  end  and  the  joints 
or  the  whole  interiors  of  the  sections  asphalted  as  they  are  laid. 


38o 


ELECTRIC    TRANSMISSION   OF  POWER. 


The  following  table  gives  the  properties  of  steel  hydraulic 
pipe  of  the  sizes  in  common  use,  and  double  riveted: 


Diameter  in  inches  

16 

18 

36 

Area  in  square  inches.  .  . 

78" 

"3 

T53 

2OI 

254 

3*4 

452 

706 

1,017 

1,385 

Cubic  feet  per  minute  at 
three  feet  per  second.  . 

TOO 

142 

200 

2S5 

320 

400 

57° 

890 

1,300 

1,760 

Weight     in   pounds    per 
foot  

26 

06.? 

67 

74-5 

Safe  head  in  feet  

000 

750 

650 

560 

500 

45° 

375 

300 

150 

135 

Change  in  sate  head  for 
each  gauge  number... 

100 

90 

80 

70 

60 

55 

45 

35 

20 

20 

The  pipe  is  assumed  to  be  of  No.  10  gauge  steel,  and  the 
changes  in  safe  head  are  of  course  approximate  only,  but  hold 
with  sufficient  exactness  for  a  variation  of  four  or  five  gauge 
numbers.  It  is  better  to  use  a  pipe  too  thick  than  one  too 
thin,  and  to  use  extra  heavy  pipe  at  bends.  Where  the  ground 
permits,  the  water  can  often  be  carried  to  advantage  in  a  flume 
or  ditch,  and  then  dropped  through  a  comparatively  short  pipe 
line.  For  heads  approaching  or  surpassing  1,000  feet  it  is  prob- 
ably safer  to  use  lap-welded  tube  for  the  lower  portion  of  the 
run.  In  every  case  suspended  sand  must  be  kept  out  of  the 
water,  else  it  will  cut  the  wheels  and  nozzles  like  a  sand  blast. 
When  one  remembers  that  under  400  feet  head  the  spouting 
velocity  of  the  water  is  about  160  feet  per  second,  the  need  of 
this  precaution  is  evident.  A  large  settling  tank  is  usually 
provided  at  the  head  works,  spacious  and  deep  enough  to  let 
the  pipe  draw  from  the  clear  surface  water.  At  its  lower  end 
the  pipe  line  terminates  in  a  receiver — a  heavy  cylindrical  steel 
tank  of  considerably  larger  diameter  than  the  pipe  proper, 
from  which  water  is  distributed  to  the  wheels. 

On  very  high  heads  a  relief  valve  is  attached  at  or  near  the 
receiver  to  avert  danger  from  a  sudden  increase  in  pressure  in 
the  pipe,  such  as  might  be  caused  by  some  sudden  obstruction 
at  the  gate. 

This  pipe  line  method  of  supply  is  considerably  used  for 
turbines  of  moderate  size  on  heads  as  low  as  75  to  100  feet,  in 
cases  where  the  natural  fall  of  the  stream  is  rather  sudden.  It 
really  amounts  to  a  considerable  elongation  of  the  iron  pen- 
stock which  is  in  common  use.  Whenever  there  is  a  sharp 


HYDRA  ULIC  DE  VELOPMENT.  31 

declivity  in  difficult  country,  piping  is  often  easier  and  cheaper 
than  constructing  a  sinuous  flume  or  canal.  In  such  situa- 
tions the  pipes  may  be  5  or  6  feet  in  diameter  or  even  more, 
and  being  under  very  moderate  pressure,  may  be  comparatively 
light  and  cheap. 

In  cold  climates  ice  is  one  of  the  difficulties  most  to  be 
dreaded  in  hydraulic  work.  In  high  pressure  pipe  lines  there 
is  little  to  fear,  for  fast-running  water  does  not  freeze  easily 
and  the  pipes  can  generally  be  readily  covered,  as  in  the  San 
Antonio  Canon  plant,  enough  to  prevent  freezing.  Large 
canals  simply  freeze  over  and  the  interior  water  is  thus  pro- 
tected. But  in  cold  climates  there  is  considerable  danger  of 
the  so-called  anchor-ice.  This  is,  in  extremely  cold  weather, 
formed  on  the  bed  and  banks  of  rapid  and  shallow  streams. 
The  surface  does  not  freeze,  but  the  water  is  continually  on 
the  point  of  freezing  and  flows  surcharged  with  fine  fragments 
of  ice  that  pack  and  freeze  into  a  solid  mass  with  the  freezing 
water  rapidly  solidifying  about  it.  When  in  this  condition  it 
rapidly  clogs  the  racks  that  protect  the  penstocks,  and  even 
the  wheel  passages  themselves.  In  extremely  cold  climates 
under  similar  circumstances  the  water  becomes  charged  with 
spicular  ice  crystals  known  as  frazil  in  Canada,  far  worse  to 
contend  with  than  ordinary  anchor  ice. 

The  best  protection  against  ice  is  a  deep,  quiet  pond 
above  the  dam,  in  which  no  anchor  ice  can  form,  and  which 
will  attach  to  its  own  icy  covering  any  fragments  that  drift 
down  from  above.  In  case  of  trouble  from  anchor  ice,  about 
the  only  thing  to  do  is  to  keep  men  working  at  the  racks  wjth' 
long  rakes,  preserving  a  clear  passage  for  the  water.  If  the 
wheel  passages  begin  to  clog  there  is  no  effective  remedy. 

The  most  dangerous  foe  of  hydraulic  work  is  flood.  The 
precautions  that  can  be  taken  are,  first,  to  have  the  dam  and 
head-works  very  solid,  and  second,  so  to  locate  them  if  possible 
as  to  have  an  adequate  spillway  over  which  even  a  very  large 
amount  of  surplus  water  can  flow  without  endangering  the 
main  works.  If  a  pipe  line  is  used  it  must  be  laid  above  high 
water  mark,  else  the  first  freshet  will  probably  carry  it  away. 
The  power  station  must  likewise  be  out  of  reach  even  of  the 
highest  water. 

Closely  connected  with  the  subject  of  floods  is  that  of  varia- 


382  ELECTRIC  TRANSMISSION  OF  POWER. 

ble  head,  which  in  many  streams  is  a  constant  source  of  diffi- 
culty. In  times  of  flood  the  extra  height  of  the  water  above 
the  dam  is  generally  useless,  while  the  tail  water  rises  and 
backs  up  into  the  wheels,  cutting  down  their  power  and  speed, 
often  very  seriously.  This  matter  has  already  been  discussed 
in  Chapter  IX.,  in  so  far  as  it  is  connected  with  the  arrange- 
ment of  the  turbines.  At  very  high  heads  this  trouble  van- 
ishes, as  no  possible  variation  of  the  water  level  can  be  a 
considerable  fraction  of  the  total  head. 

The  most  delicate  questions  involved  in  hydraulic  develop- 
ment are  those  connected  with  variable  water  supply.  Having 
ascertained  as  nearly  as  possible  the  minimum  flow,  the  mini- 
mum natural  continuous  supply  of  power  is  fixed,  but  it  remains 
to  be  determined  how  the  water  in  excess  of  this  shall  be 
utilized,  if  at  all. 

Three  courses  are  open  for  increasing  the  available  mer- 
chantable power.  First,  water  can  be  stored  to  tide  over  the 
times  of  small  natural  supply.  Second,  a  plant  can  be  installed 
to  utilize  what  water  is  available  for  most  of  the  year  and  can 
be  curtailed  in  its  operation  during  the  season  of  low  water. 
Third,  the  service  can  be  made  continuous  by  an  auxiliary 
steam  plant  in  the  power  station.  Storage  of  water  can 
obviously  be  used  in  connection  with  either  of  the  other 
methods. 

Under  very  high  heads  storage  is  generally  worth  the  while 
if  the  lay  of  the  land  is  favorable.  This  of  course  means  a 
dam,  but  not  necessarily  a  very  high  or  costly  one.  If  possible 
the  storage  reservoir  should  be  a  little  off  the  main  flow  of  the 
stream  so  as  to  escape  damage  from  freshets.  Reverting  to 
our  previous  example  of  storage,  suppose  we  have  500  HP 
available  easily  for  nine  months  of  the  year,  but  a  strong 
probability  of  not  over  250  HP  for  the  remaining  three  months. 
We  have  already  seen  that  under  these  circumstances  15 
acres  flooded  10  feet  deep  will  keep  up  the  full  supply  for  a 
month.  If  say  50  acres  can  be  thus  flooded,  the  all-the-year- 
round  capacity  of  the  plant  will  be  doubled.  In  the  moun- 
tainous localities  where  such  heads  are  to  be  found,  land  has 
usually  only  a  nominal  value,  and  impounding  the  equivalent 
of  this  amount  of  water  is  frequently  practicable.  If  it  can  be 
done  at  say  a  cost  of  $75,000,  the  annual  charge  per  HP  stored, 


HYDRAULIC  DEVELOPMENT.  383 

counting  interest  and  sinking  fund  at  10  per  cent.,  will  be  $30, 
and  the  investment  would  generally  be  a  profitable  one.  If 
the  storage  cost  $100,000,  the  annual  charge  would  be  $40, 
and  this  would  not  infrequently  be  well  worth  the  while,  when 
power  could  be  sold  for  a  good  price. 

At  lower  heads  the  annual  charge  per  HP  stored  would  be 
considerably  greater  for  the  sam?  total  expenditure.  Some- 
times, however,  storage  capacity  can  be  much  more  cheaply 
gained  for  both  high  and  low  heads,  at  for  instance  not 
more  than  half  the  charge  just  mentioned.  The  matter  is 
always  worth  investigating  thoroughly  when  there  is  doubt 
about  supplying  the  power  market  with  the  natural  flow.  The 
points  to  be  looked  into  are  the  nature  and  extent  of  the  low 
water  period,  and  the  cost  of  developing  various  amounts  of 
storage  capacity.  Sometimes  the  period  of  extreme  low  water 
is  much  shorter  than  that  assumed,  and  storage  is  correspond- 
ingly cheaper. 

There  are  some  cases  in  which  it  is  possible  to  supply  cus- 
tomers with  power  for  nine  or  ten  months  in  the  year,  falling 
back  on  the  individual  steam  plants  in  the  interim.  When 
transmitted  power  can  be  cheaply  had,  it  is  worth  while  for 
the  power  user  who  is  paying,,  say  $100  per  HP  per  year  for 
steam  power  to  take  electric  power  at  $50  per  HP  per  year  for 
nine  months,  and  to  use  steam  the  other  three  months.  Certain 
industries,  too,  are  likely  to  be  comparatively  inactive  in  mid- 
summer, or  may  find  it  worth  while  to  force  their  output 
during  the  months  when  cheap  power  is  obtainable,  and  shut 
down  or  run  at  reduced  capacity  when  the  power  is  unavail- 
able. This  is  a  matter  very  dependent  on  local  conditions, 
and  while  the  demand  for  such  partial  power  supply  is  gener- 
ally limited,  there  are  many  cases  in  which  it  would  be  advan- 
tageous for  all  parties  concerned.  In  some  mountainous 
regions,  winter  is  the  season  of  low  water  owing  to  freezing, 
and  various  industries  are  suspended  which  may  be  profitably 
supplied  with  power  when  the  winter  unlocks  its  gates. 

Eking  out  the  water  supply  by  an  auxiliary  steam  power 
station  is  likewise  not  of  general  applicability,  but  sometimes 
may  prove  advantageous.  It  is  most  likely  to  prove  useful  in 
localities  where  a  steam  power  plant  would  pay  by  virtue  of 
the  economy  due  to  production  on  a  large  scaje  and  distribu- 


384  ELECTRIC    TRANSMISSION  OF  POWEk. 

tion  to  small  users.  Cheap  water  power  a  large  part  of  the 
year  then  abundantly  justifies  adjunct  steam  power  when 
necessary.  The  moral  effect  of  continuous  power  supply 
is  valuable  m  securing  a  market.  Whether  such  a  supply  is 
profitable  depends  on  the  ratio  between  the  cost  of  water 
power  and  the  cost  of  steam  power.  And  it  must  not  be  for- 
gotten that  steam  power  for  two  or  three  months  in  the  year  is 
relatively  much  more  costly  than  continuous  power. 

The  general  charges  are  the  same,  although  labor,  coal,  and 
miscellaneous  supplies  decrease  nearly  as  the  period  of  opera- 
tion.  Consequently,  since  there  is  this  large  fixed  item, 
amounting  to  from  20  to  40  per  cent,  of  the  total  annual  cost, 
the  cost  of  pover  in  a  plant  operated  only  three  months  will  be 
relatively  at  least  50  per  cent,  greater  than  if  it  were  in  con- 
stant operation.  There  must  be  a  large  margin  in  favor  of 
water  power  to  justify  this  auxiliary  use  of  steam,  unless  the 
latter  would  pay  on  its  own  account,  as  for  instance  in  a  plant 
used  largely  for  lighting,  which  would  be  the  most  profitable 
kind  of  electric  service  were  there  a  sufficiently  large  market. 
Moreover  a  large  proportion  of  lighting  automatically  relieves 
the  load  in  midsummer,  the  most  usual  time  of  low  water. 

The  Actual  economics  of  such  a  question  can  only  be  deter- 
mined after  a  thorough  examination  of  local  costs,  since  the 
costs  of  both  steam  and  water  power  have  a  wide  range. 

Steam  power  on  a  12  hour  basis  at  steady  full  lead  varies 
according  to  the  size  and  kind  of  plant,  cost  of  fuel,  and  so 
forth,  from  a  little  under  $20  per  HP  year  to  $125  or  more, 
with  an  increase  of  one-third  to  one-half  in  case  of  variable 
loads.  Water  power  fully  developed  rents  for  from  $5  to  $50 
or  more  per  HP  year,  and  may  cost  to  develop  anywhere  from 
$20  to  $150  per  HP.  At  the  former  price  it  is  cheaper  than 
steam  under  any  circumstances,  at  the  latter  it  is  dearer  than 
steam  unless  the  fuel  cost  is  abnormally  high. 

If  the  cost  of  hydraulic  development  can  be  kept  below 
$100  per  HP,  water  power  can  nearly  always  drive  steam  power 
out  of  business. 

With  respect  to  the  prime  movers  to  be  employed  in  a 
hydraulic  development  one  must  be  governed  largely  by  cir- 
cumstances. The  choice  in  general  lies  between  turbines  and 
impulse  wheels,  the  properties  of  which  have  been  fully  dis« 


HYDRAULIC  DEVELOPMENT.  385 

cussed  in  Chapter  IX.  Without  attempting  to  draw  any  hard 
and  fast  lines,  turbines  are  preferable  up  to  about  100  feet  head, 
unless  very  low  rotative  speed  is  desirable,  or  very  little  power 
is  to  be  developed.  Above  that,  the  impulse  wheels  grow 
more  and  more  desirable,  and  at  200  feet  head  the  whole  field  is 
practically  their  own.  It  is  generally  practicable  and  desirable 
to  use  wheels  with  a  horizontal  axis.  Only  in  a  few  instances  is 
it  necessary  to  resort  to  a  vertical  axis,  as  when  there  is  consid- 
erable danger  of  the  tail  water  rising  clear  up  to  the  wheels  or 
when,  as  at  Niagara,  a  very  deep  wheel  pit  is  employed. 

The  line  of  operations  in  developi-ng  a  water  power  subse- 
quent to  the  reconnaissance  has  already  been  indicated.  After 
the  more  general  considerations  have  been  determined,  comes 
the  question  of  utilization. 

It  may  seem  needless  to  suggest  that  the  first  thing  neces- 
sary is  an  actually  available  market,  but  the  author  has  more 
than  once  had  imparted  to  him,  under  pledge  of  solemn  secrecy, 
the  location  of  "magnificent"  water  powers  which  could  be 
developed  for  a  mere  song,  located  a  hundred  miles  from 
nowhere — out  of  effective  range  even  of  electrical  transmission. 

Having  a  possible  market,  the  next  thing  is  to  investigate 
it  thoroughly.  The  actual  amount  of  steam  power  must  be 
found,  together  with  its  approximate  cost  in  large  and  in 
small  units.  This  information  ought  to  be  extended  to  at 
least  an  approximate  list  of  every  engine  used  and  the  nature 
of  its  use,  whether  for  constant  or  variable  load,  whether  in 
use  throughout  the  year  or  only  at  certain  seasons.  These 
more  minute  data  are  not  immediately  necessary,  but  are 
immensely  useful  later.  If  it  is  proposed  to  include  electric 
lighting  in  the  scheme  a .}  estimate  of  the  probable  demand 
for  lights  should  be  carefully  made.  A  fair  guess  at  this  can 
be  made  from  the  number  of  inhabitants  in  the  city  or  town 
supplied.  Where  there  is  competition  only  with  gas,  experi- 
ence shows  that  the  total  number  of  incandescent  lights  installed 
is  likely  to  be,  roughly,  from  one-fourth  to  one-sixth  of  the 
population,  occasionally  as  many  as  one-third,  or  as  few  as 
one-eighth.  In  cities  of  moderate  size  it  is  usually  found  that 
even  with  competition  from  gas  the  annual  sales  of  electricity 
for  all  purposes  can  with  proper  exploitation  be  brought  up  to 
from  $1.50  to  $2. oo per  capita.  This  amount  may  be  increased 
by  50  per  cent,  under  favorable  conditions. 


386  ELECTRIC    TRANSMISSION  OF  POWER. 

From  the  data  thus  obtained  one  can  estimate  the  general 
size  of  the  market,  and  hence  the  approximate  possible  demand 
for  electrical  energy.  With  this  in  mind  further  plans  for  the 
hydraulic  development  can  be  made.  It  may  be  that  the 
water  power  is  obviously  too  small  to  fill  the  market,  if  so,  it 
should  be  developed  completely.  If  not,  much  judgment  is 
necessary  in  determining  the  desirable  extent  of  the  develop- 
ment. Probable  growth  must  be  taken  into  account,  but  iu 
cannot  safely  be  counted  upon.  If  steam  power  is  very 
expensive  most  of  the  engines  can  probably  be  replaced  by 
motors.  The  replacement  of  one-half  of  them  is,  under 
average  circumstances,  a  sufficiently  good  tentative  estimate. 

With  this  as  a  basis  approximate  estimates  of  the  hydraulic 
development  can  be  made.  This  should  be  done  by  a  compe- 
tent hydraulic  engine'er.  If  the  development  is  easy  it  is  well 
to  make  estimates  for  a  liberal  surplus  power  also.  At  this 
stage  it  is  best  to  have  the  hydraulic  and  the  electrical  engi- 
neer work  hand  in  hand  to  estimate  on  the  delivery  of  the 
assumed  amount  of  power.  From  these  estimates  the  general 
outlook  for  returns  can  be  reckoned. 

Before  actually  beginning  work  it  is  advisable  to  make  a 
pretty  thorough  preliminary  canvass  of  the  market  to  see 
what  can  be  done  immediately  in  the  sale  of  power  and  light. 
With  the  certain  and  the  probable  consumption  ascertained, 
the  hydraulic  and  electrical  engineers  can  work  their  plans 
into  final  shape  and  prepare  final  estimates^ 

All  this  preliminary  work  may  at  first  sight  seem  rather 
unnecessarily  exhaustive,  but  mistakes  on  paper  are  corrected 
more  easily  than  any  others,  and  the  investigation  is  likely  to 
save  many  times  its  cost  in  the  final  result. 

Whatever  is  done  should  be  done  thoroughly.  Poor  work 
seldom  pays  anywhere,  least  of  all  in  a  permanent  installation, 
and  it  should  be  conscientiously  avoided. 

Above  all,  continuity  of  service  has  a  commercial  value  that 
cannot  be  estimated  from  price  lists.  If  it  anywhere  pays  to 
be  extravagant,  it  is  in  taking  extreme  precautions  against 
breakdowns  and  in  facilities  for  quick  and  easy  repairs  in  case 
of  unavoidable  accident.  This  applies  alike  to  the  hydraulic 
and  the  electrical  work.  If  the  first  severe  freshet  demoral- 
izes the  hydraulic  arrangements,  or  the  plant  runs  short  of 


HYDRAULIC  DEVELOPMENT.  387 

water  at  the  first  severe  drought,  a  damage  is  done  that  it 
takes  long  to  repair  in  the  public  mind.  On  the  other  hand 
careful,  thorough  work,  coupled  with  intelligent  foresight, 
insures  that  complete  reliability  that  is  the  mint  mark  of 
honest  and  substantial  enterprises. 


CHAPTER  XL 

THE    ORGANIZATION    OF    A    POWER   STATION. 

THE  first  thing  to  be  determined  in  planning  a  power  station 
is  the  proper  site,  which  should,  if  steam  be  the  motive  power, 
be  settled  by  convenience  with  respect  to  the  supply  of  coal 
and  water.  In  using  water  power  the  position  of  the  station 
should  be  determined  in  connection  with  the  hydraulic  devel- 
opment. The  foot  of  the  working  fall  is  the  natural  site,  but, 
particularly  in  mountainous  regions,  it  may  be  quite  unavail- 
able on  account  of  lack  of  available  space,  unsuitable  ground 
for  foundations,  inaccessibility,  or  more  often  danger  of  flood. 
Under  high  heads  where  a  pipe  line  is  used,  one  has  a  con- 
siderable amount  of  freedom  in  determining  the  site,  since  the 
pipe  can  be  extended  and  led  around  to  convenient  locations 
at  moderate  expense,  say  not  more  than  $3  or  $4  per  foot. 
A  relatively  small  sacrifice  of  head,  too,  may  enable  one  to 
secure  an  admirable  location. 

On  low  heads  there  is  far  less  latitude  permissible,  since  the 
canal  and  tail  race  are  relatively  costly  and  a  change  of  level 
is  a  serious  matter. 

If  possible  the  power  station  should  be  placed  well  off  the 
main  line  of  flow,  or  with  the  main  floor  well  above  high  water 
mark.  The  foundations  must  be  of  the  best  to  secure  safety 
from  floods  and  a  proper  support  for  the  moving  machinery. 
To  meet  these  conditions  is  not  always  easy,  particularly 
when  the  available  head  is  low,  and  sometimes  extreme  artificial 
precautions  have  to  be  taken  against  flood.  Such  a  case  is 
found  in  the  Oregon  City  plant  already  mentioned,  of  which  a 
sectional  view  is  ^iven  in  Fig.  214,  showing  the  foundations,  a 
single  generate"  its  wheels,  and  their  appurtenances.  The 
inner  wall  of  the  station  is  here  the  outer  wall  of  the  canal, 
and  both  walls  and  foundations  are  built  very  solidly  of 
masonry  and  concrete.  In  the  cut  A  and  B  are  the  draft 
tubes  belonging  respectively  to  the  wheel  cases  D  and  F,  which 

388 


THE   ORGANIZATION  OF  A   POWER   STATION. 


FIG.  214. 


390  ELECTRIC   TRANSMISSION  OF  POWER. 

are  supplied  by  the  penstocks  C  and  E.  F  contains  the 
regular  service  turbine,  a  42"  Victor  wheel  coupled  direct  to 
the  generator  at  P.  On  the  pedestals  G  above  this  wheel  is  a 
ring  thrust  bearing  at  /  and  an  hydraulic  thrust  bearing  K. 
Above  this  is  a  pulley  F,  6  feet  in  diameter,  and  still  above 
this  the  upper  bearing  support,  the  bearings  N  and  O,  the 
coupling  M  and  pedestals  Q. 

The  wheel  case  D  contains  a  60"  wheel  with  bearings,  pulley, 
and  so  forth,  R,  S,  Wt  T,  U.  The  function  of  this  wheel  and 
its  attachments  is  to  supply  power  at  the  seasons  of  very  high 
water,  sometimes  several  years  apart.  When  the  tail  water 
backs  up  so  far  that  the  smaller  wheel  is  no  longer  equal  to  the 
work  the  generator  shaft  is  arranged  to  be  uncoupled  just 
above  the  wheel.  Then  the  belt  tightener  X  can  be  brought 
into  use,  the  large  wheel  started,  and  the  generator  driven  by 
the  horizontal  belt.  The  belt  tightener  is  operated  by  hand 
wheels  at  E^  and  Z>2,  while  similar  hand  wheels  at  C2  and  B^ 
enable  the  wheels  to  be  regulated  by  hand  when  desirable. 
The  governing  is  normally  accomplished  by  the  automatic 
regulator  Az.  F^  is  one  of  the  main  race  gates,  lifted  by  the 
mechanism  at  G^,  The  wheelroom  is  lighted  by  water-tight 
heavy  glass  bulls  eyes  at  Z,  each  three  feet  in  diameter.  The 
dynamo  room  is  lighted  by  side  windows  and  monitor  roof  and 
is  fitted  with  a  twelve  ton  travelling  crane  K^  carried  on  the 
supporting  column  M^  and  Nf  The  penstocks  pass  through 
the  heavy  cement  floor  of  the  wheelroom,  J^  with  water-tight 
joints.  The  main  point  of  interest  in  this  station  for  our  pres- 
ent purpose  is  not  the  somewhat  complicated  and  cumbersome 
hydraulic  plant  but  the  structure  of  the  wheelroom,  which 
forms  a  massive  permanent  coffer  dam  securing  the  motive 
power  against  all  direct  interference  by  even  the  fiercest 
floods.  Such  a  construction  is  somewhat  inconvenient,  but  in 
some  instances  is  almost  absolutely  necessary.  The  design  of 
this  plant  is  unique,  in  some  respects  uniquely  bad  from  the 
standpoint  of  general  practice,  but  most  of  its  peculiarities  are 
the  result  of  its  situation  and  of  unusual  conditions  of  water 
supply  which  forced  the  use  of  uncommon  remedies.  Generally 
such  extreme  measures  need  not  be  taken,  although  since  it  is 
usually  desirable  to  have  the  dynamos  on  a  level  with  the 
wheels,  and  coupled  to  them,  a  water-tight  wall  between  the 


THE   ORGANIZATION   OF  A    POWER    STATION. 


391 


dynamo  room  and  the  wheelroom  is  rather  common.  Quite 
as  often,  however,  full  reliance  is  placed  on  the  strength  and 
tightness  of  the  penstocks  and  wheel  cases,  and  wheels  and 
dynamos  are  placed  in  the  same  room.  A  plant  so  arranged 
is  cheap  and  simple,  and  where  there  is  no  unusual  danger 


FIG.  215. 

of  flood  is  sufficiently  secure.  Fig.  215  shows  a  good  typical 
plant  of  this  sort,  consisting  of  three  double  horizontal  tur- 
bines under  50  feet  head,  each  directly  coupled  to  its  generator. 
Each  pair  of  wheels  gives  560  HP  at  about  430  revolutions  per 
minute.  This  represents  construction  as  straightforward  and 
simple  as  that  of  Fig.  214  was  difficult  and  intricate.  It  is 
specially  interesting  for  the  arrangement  of  several  wheels  to 
discharge  into  a  common  tailrace,  instead  of  into  several 
costly  arched  tailraces  extending  under  the  dynamo  room,  a 
construction  sometimes  unnecessarily  employed. 

The  hydraulic  conditions  may  drive  the  engineer  to  all  sorts 


392 


ELECTRIC    TRANSMISSION  OF  POWER. 


of  expedients,  but  the  main  points  are  security  against  being 
drowned  out,  and  good  foundations.  If  the  dynamos  and 
wheels  can  be  given  direct  foundations  of  masonry  and  con- 
crete, such  as  the  former  have  in  Fig.  215  and  the  latter  in  Fig. 
214,  so  much  the  better.  If  moving  machinery  must  be  carried 
on  beams,  support  these  beamsas  in  Fig.  215,  directly  under  the 
load,  by  iron  pillars  or  masonry  piers.  For  direct  coupling  it 
is  preferable  to  have  foundations  entirely  secure  from  vibration. 


TRANSFORMER  HOUSE 


CANAL 


FIG.  216. 

If  such  cannot  be  had  one  may  resort  successfully  to  a  flexible 
coupling,  but  more  often  rope  or  belt  driving  is  advisable. 

The  proper  site  having  been  selected  the  next  consideration 
is  the  form  of  the  structure  itself.  As  a  rule,  whatever  the 
nature  of  the  power  units,  they  are  most  conveniently  put,  in 
a  water  power  plant,  side  by  side  in  a  single  row  with  their 
shafts  parallel.  This  placing  enables  the  hydraulic  plant  to  be 
simply  and  conveniently  arranged,  and  enables  the  operator  to 
take  in  the  whole  plant  at  a  glance  and  watch  all  the  apparatus 
simultaneously.  Fig.  216  shows  the  original  ground  plan  of 
the  great  Niagara  station,  well  exemplifying  this  arrangement. 


THE   ORGANIZATION  OF  A    POWER   STATION.         393 

In  stations  employing  horizontal  turbines  such  a  distribu- 
tion of  units  has  even  greater  advantage  in  avoiding  long  and 
crooked  penstocks.  Fig.  215  forcibly  suggests  the  difficulty  of 
setting  the  generators  otherwise  than  in  a  single  row. 

In  general  the  building  erected  for  a  power  station  should  be 
light,  dry,  fireproof,  and  well  ventilated.  Dynamos  usually 
run  hot  enough,  without  boxing  them  up  in  a  close  room. 
There  should  be  plenty  of  space  back  of  the  row  of  dynamos  so 
that  if  machinery  has  to  be  moved  there  will  be  ample  room. 
On  the  other  hand  the  row  of  dynamos  should  be  fairly  compact, 
as  a  needless  amount  of  scattering  of  the  machines  makes  them 
hard  to  look  after.  In  very  many  cases  one  story  in  height  is 
quite  sufficient,  and  in  all  cases  it  is  preferable  to  more,  so  far 
as  working  apparatus  is  concerned.  Sometimes  a  second  story 
can  be  well  utilized  for  store  rooms,  transformer  room,  and 
quarters  for  the  operating  force,  but  as  a  rule  a  single  story 
allows  more  complete  accessibility — one  of  the  most  important 
features  in  station  design.  As  land  is  seldom  dear  around  a 
station  for  power  transmission  ample  floor  space  is  easily 
obtained,  except  in  occasional  cramped  localities.  A  brick 
structure  with  iron  roof  is  perhaps  the  most  satisfactory  kind 
of  station,  although  for  small  work  a  frame  building  often 
answers  the  purpose.  Window  space  should  be  large  and 
arranged  so  as  to  avoid  leaving  dark  corners  around  the 
apparatus.  There  should  be,  too,  ample  door  space  to  facili- 
tate replacing  apparatus — nothing  is  more  annoying  than  to 
be  short  of  elbow  room  when  moving  heavy  machinery. 

For  the  same  reason  a  good  permanent  road  should  be  built 
to  the  power  station  if  one  is  not  already  in  existence.  In 
mountainous  regions  this  is  sometimes  impracticable,  but 
money  spent  in  improving  the  road  is  better  invested  than 
when  put  into  special  sectionalized  apparatus.  It  is  quite 
possible  so  to  sectionalize  a  generator  of  several  hundred 
KW  that  the  parts  can  all  be  carried  on  mule  back,  but  the 
expense  is  considerably  increased,  and  the  great  advantage  of 
having  a  standard  type  of  apparatus  has  to  be  abandoned. 
Hence,  unless  the  cost  of  improving  the  road  to  admit  of  trans- 
porting ordinary  apparatus  is  decidedly  greater  than  the  differ- 
ence in  cost  between  regular  and  sectionalized  machinery,  the 
former  procedure  is  advisable.  Of  course  when  it  comes  to  a 


394  ELECTRIC    TRANSMISSION  OF  POWER. 

question  of  long  mountain  trails,  sectionalized  machinery  has 
to  be  employed.  The  armature  of  a  polyphase  machine  for 
use  with  transformers  can  very  easily  be  sectionalized,  but  if 
for  high  voltage  or  of  very  large  size  it  is  better  to  send  in  core 
plates  and  other  material  in  bundles  and  wind,  the  armature  on 
the  spot. 

Having  determined  the  general  location  and  nature  of  the 
power  station,  one  may  take  up  further  arrangements  as 
follows: 

I.  Motive  Power. 

II.  Dynamos. 

III.  Transformers. 

IV.  Accessories. 

The  fundamental  question  is  the  proper  size  and  character 
^of  power  units.  In  direct  coupled  work  prime  mover  and 
generator  must  be  considered  together.  In  steam  driven 
stations  for  power  transmission  the  boiler  plant  may  be 
determined  by  itself,  but  dynamos  and  engines  should  be  taken 
up  conjointly. 

There  is  at  present  rather  top  strong  a  general  inclination 
.to  use  direct  coupled  units  at  any  cost.  Direct  driving  is 
beautifully  simple  and  efficient  when  conditions  are  favorable, 
but  belt  and  rope  driving  gives  singularly  little  trouble,  and 
when  well  engineered  wastes  very  little  energy — not  over  3  to 
5  per  cent,  for  a  single  direct  drive,  which  can  almost  invaria- 
bly be  used.  It  is  very  easy  to  lose  far  more  than  this  in 
using  a  dynamo  designed  for  a  speed  unsuited  for  its  output, 
or  wheels  working  under  disadvantageous  conditions.  Cases 
of  such  misfit  combinations  are  not  uncommon,  and  while  the 
workmanship  and  results  are  often  good  the  engineering  is 
faulty.  A  very  characteristic  example  is  shown  in  Fig.  217, 
from  the  power  plant  of  a  synchronous  single  phase  transmis- 
sion for  mining  purposes.  The  generator  selected  was  a  120 
KW  Westinghouse  machine  of  standard  form  and  excellently 
adapted  for  its  purpose.  Its  speed  was  860  revolutions  per 
minute,  and  to  obtain  this  from  a  working  head  of  340  feet  a 
battery  of  four  21  inch  Pelton  wheels  was  required.  Now  the 
Pelton  wheel  under  favorable  conditions  is  unexcelled  as  a 
prime  mover  in  convenience  and  efficiency,  but  these  conditions 
were  distinctly  unfavorable.  The  same  work  could  have  been 


THE   ORGANIZATION  OF  A    POWER   STATION.          395 

done  by  a  single  wheel  four  or  five  feet  in  diameter  at  not  over 
one-third  the  initial  expense  for  wheels  and  fittings,  and  at 
enough  higher  efficiency  to  more  than  compensate  for  the 
slight  loss  of  energy  in  a  simple  belt  drive.  In  this  case  wheel 
efficiency  was  sacrificed  to  the  speed  of  the  generator.  An 
error  quite  as  common  is  to  sacrifice  generator  efficiency  to 
the  speed  of  the  prime  mover. 

The  most  flagrant  case  of  this  kind  that  has  come  to  the 
author's  notice  was  a  polyphase  machine  of  less  than  a  hundred 
KW  output  direct  coupled  to  a  vertical  shaft  turbine  at  20 
revolutions  per  minute.  This  was  of  course  a  low  frequency 
machine,  but  an  instance  nearly  as  bad  may  be  found  in  the 
case  of  a  75  KW  alternator  for  15,000  alternations  per  minute 
direct-coupled  to  an  engine  at  a  little  less  than  100  revolutions 
per  minute.  These  are  extreme  examples,  of  course,  such 
machines  costing  several  times  more  than  normal  generators 
of  the  same  capacity,  and  having  probably  fully  10  per  cent, 
less  efficiency.  It  is,  however,  not  rare  to  find  costly  direct 
coupled  units  which  gain  no  efficiency  over  belted  combina- 
tions, have  little  to  recommend  them  save  appearance,  and  pay 
dearly  for  that. 

The  best  way  to  avoid  such  mistakes  is  to  put  aside  preju- 
dice and  let  the  makers  of  generators  and  prime  movers  put 
their  heads  together  in  consultation  and  work  out  the  problem 
together.  Both  are  usually  anxious  to  do  good  work,  and  will 
arrive  at  a  judicious  conclusion. 

Alternating  work  is  sometimes  difficult  in  this  respect  on 
account  of  the  requirements  of  frequency,  but  at  the  present 
time  all  the  large  makers  of  hydraulic  and  electrical  machinery 
have  a  sufficient  line  of  patterns  to  meet  most  cases  easily 
without  involving  special  work  to  any  considerable  extent. 

In  deciding  on  the  number  of  units  to  be  employed  several 
things  must  be  taken  into  account.  The  number  should  not 
be  so  small  that  the  temporary  crippling  of  a  single  unit  will 
interfere  seriously  with  the  work  of  the  plant.  This  deter- 
mines the  maximum  permissible  size  of  -each  unit.  The 
nearer  one  can  come  to  this  without  involving  difficulties  in 
the  way  of  proper  speed  or  serious  specialization,  the  better. 
It  is  seldom  advisable  to  install  less  than  three  units,  while  in 
some  cases  a  considerably  larger  number  must  be  used  to  suit 
the  hydraulic  conditions. 


396  ELECTRIC    TRANSMISSION  OF  POWER. 


THE   ORGANIZATION  OF  A   POWER  STATION.         397 

To  illustrate  this  point,  suppose  we  are  considering  a  trans- 
mission of  3,000  KW  from  a  water  power  with  16  feet  available 
head.  One  would  naturally  like  to  install  three  1,000  KW 
generators  or  four  of  750  KW.  But  trouble  is  encountered  at 
once  in  the  wheels.  The  1,000  KW  machine  should  have  say 
1,500  HP  available  at  the  wheel,  and  the  750  KW  about  1,100. 
Even  assuming  at  once  the  use  of  double  turbines  the  highest 
available  speed  for  an  output  of  1,500  HP  would  be  about  75  to- 
80  revolutions  per  minute,  too  low  for  advantageous  direct 
coupling  at  any  ordinary  frequency;  1,100  HP  can  be  obtained 
at  a  speed  perhaps  10  revolutions  per  minute  higher — not 
enough  to  be  of  much  service.  It  is  a  choice  between  evils  at 
best,  either  generators  of  speed  so  low  as  to  be  both  expensive 
and  difficult  to  get  up  to  normal  efficiency,  or  belting,  when  one 
would  much  prefer  to  couple  direct.  At  lower  heads,  say  12 
feet,  one  would  be  driven  from  direct  connection;  at  30  to  40 
feet  head  it  would  be  comparatively  easy.  In  the  case  in  hand 
we  are  near  the  dividing  line,  and  it  would  require  very  close 
figuring  to  get  at  the  real  facts,  figuring  which  would  have  to 
be  guided  by  local  conditions.  The  chances  are  that  rope 
driven  generators  of  perhaps  750  KW  would  give  the  best 
combination  of  efficiency  and  cost.  As  an  alternative  one 
might  use  six  500  KW  generators  at  about  125  r.p.m.,  which 
would  not  be  bad.  Each  case  of  this  kind  has  to  be  worked 
out  on  its  merits.  Since  the  dynamos  cost  far  more  than 
water  wheels  for  the  same  capacity,  if  there  is  any  specializing 
to  be  done  it  is  cheaper  to  do  it  at  the  wheels.  If,  however, 
it  proves  convenient  to  change  the  dynamo  speed  a  trifle,  most 
generators  can  be  varied  10  per  cent,  either  way  without  en- 
countering any  difficulties. 

Now  and  then  it  becomes  necessary  to  plan  for  vertical 
wheel  shafts.  This,  unhappily,  is  apt  to  confront  one  at  very 
low  heads,  and  leads  to  immediate  difficulty.  Direct  coupling 
is  usually  impracticable  since  the  speed  is  very  low,  double 
wheels  being  out  of  the  question,  and  even  if  the  dynamo  could 
be  economically  built  the  support  of  the  revolving  element 
would  be  very  troublesome.  The  usual  arrangement  is  to  use 
bevel  gears,  and  this  is  generally  the  only  practicable  course. 
At  high  heads,  when  the  speed  is  favorable  for  direct  coupling, 
an  hydraulic  step  can  generally  be  relied  on  to  carry  the 
necessary  weight. 


398 


ELECTRIC  TRANSMISSION  OF  .POWER. 


It  is  desirable  in  any  case  to  operate  each  dynamo  by  its 
own  special  wheels,  to  avjid  complication.  Hence  the  con- 
siderations which  determine  the  number  of  dynamos  also  de- 
fine the  number  of  wheels.  It  is  very  seldom  expedient  to 


FIG.  218. 

use  more  than  a  single  pair  of  wheels  for  driving  a  single 
generator,  on  account  of  difficulties  in  alignment  and  regular 
tion  and  consequent  tendency  to  work  inharmoniously.  This- 
tendency  is  stronger  in  impulse  wheels  than  in  turbines  on 
account  of  the  very  small  volume  of  water  generally  employed,; 
and  consequent  hypersensitiveness  to  small  changes  in  the 
amount,  pressure,  and  direction  of  the  stream.  So,  usually,  a 


THE   ORGANIZATION  OF  A   POWER  STATION.        399 

single  wheel  or  pair  of  wheels,  equal  to  the  task  of  handling  a 
single  generator,  may  be  taken  as  the  hydraulic  unit. 

For  simplicity  and  economy  one  should  keep  down  the 
number  of  generators  to  the  limit  already  imposed,  except  as 
special  cases  may  call  for  an  increase.  If  the  plant  is  to  feed 
several  transmission  lines  it  is  sometimes  best  to  assign  separ- 
ate dynamos  to  each  line  for  one  purpose  or  another,  and  this 
may  make  it  necessary  to  increase  the  total  number.  The 
requisite  security  from  accident  can  be  in  such  cases  ob- 
tained by  one  or  two  spare  units,  or  by  shifting  a  generator 
from  a  lightly  loaded  line  to  a  heavily  loaded  one.  In  point 
of  fact  the  modern  generator  is  a  wonderfully  reliable  machine, 
and  it  is  not  unusual  to  find  a  machine  that  has  run  day  and 
night,  save  for  a  few  hours  in  the  week,  for  many  months  with- 
out any  reserve  behind  it.  The  author  saw  recently  a  small 
incandescent  machine  which  had  run  some  hours  per  day  in  an 
isolated  plant  for  fourteen  consecutive  years  without  a  failure 
of  any  kind.  During  that  time  the  armature  had  been  out  of 
its  bearings  but  once,  to  have  the  commutator  turned  down. 

In  steam  driven  plants,  as  in  water  power  works,  the  most 
convenient  arrangement  of  generators  is  generally  side  by 
side  in  a  single  line.  So  placed  they  are  easy  to  take  care  of, 
and  the  spare  room  is  more  available  than  when  it  is  irregularly 
disposed.  In  case  water  wheels  are  the  prime  movers  a  water- 
tight bulkhead  is  generally  placed  between  them  and  the  dyna- 
mos, so  that  leaks  or  overflows  will  be  confined  to  the  wheel 
pit,  where  they  can  do  no  harm.  Through  this  bulkhead  the 
shafts  should  pass  if  the  units  are  directly  coupled.  In  case 
of  a  belt  or  rope  drive  it  is  frequently  convenient  to  place 
wheels  and  dynamos  on  different  levels,  thus  obtaining  similar 
security.  Fig.  218  shows  a  well-arranged  small  plant  of  this 
sort,  driven  by  a  pair  of  Pelton  wheels.  The  plant  is  so  small 
that  both  dynamos  can  be  conveniently  driven  by  pulleys  on  a 
very  short  extension  of  the  wheel  shaft. 

In  a  larger  plant  each  wheel  unit  would  drive  a  single 
dynamo,  and  the  receiver  and  wheels  with  their  fittings  would 
occupy  one  half  of  the  station  while  the  dynamos  would  be 
placed  in  the  other  half,  following  the  same  general  plan  shown 
in  Fig.  218.  The  main  point  is  to  get  good  foundations  for 
the  dynamos  while  keeping  them  out  of  reach  of  stray  water. 


400  ELECTRIC    TRANSMISSION   OF  POWER. 

In  an  alternating  current  station  it  is  advisable  to  drive  the 
exciters  from  special  prime  movers,  so  that  a  change  of  speed, 
even  momentary>  in  the  main  machine  may  not  change  the 
exciter  voltage  and  thus  make  a  bad  matter  worse.  This  is  par- 
ticularly necessary  in  water  power  plants',  where  the  governing 
is  apt  to  be  none  too  close  or  prompt.  It  is  a  good  thing  also 
to  have  plenty  of  reserve  capacity  in  the  exciters,  so  as  never 
to  be  caught  with  insufficient  exciting  power  even  in  case  of 
accident  to  one  exciter. 

Both  wheels  and  dynamos  should  be  thoroughly  accessible, 
and  wheel  and  dynamo  rooms  must  be  well  lighted,  naturally 
and  artificially.  A  dark  and  slippery  wheel  pit,  without  suffi- 
cient space  around  the  wheels,  is  sure  to  prove  a  source  of 
annoyance  and  sometimes  of  serious  delays.  It  should  be 
possible,  to  get  at  every  wheel  and  its  fittings  and  to  work 
around  them  freely  when  all  the  other  wheels  are  in  full  use. 
Sometimes  it  is  useful  to  separate  wheels  by  bulkheads,  pre- 
ferably movable,  and  there  should  always  be  floor  space 
enough  to  stand  and  work  on  without  putting  up  temporary 
stagings  and  loose  boards.  There  should  always  be  electric 
lights  ready  for  use  around  all  the  working  machinery,  arc 
lamps  or  incandescents  as  may  be  most  convenient,  but  plenty 
of  them.  Around  the  wheels  it  may  sometimes  be  necessary 
to  use  incandescents  in  marine  globes  to  protect  them  from 
the  water,  and  to  install  waterproof  flexible  cable  for  the  mov- 
able lights. 

As  an  example  of  good  practice  in  a  plant  for  heavy  power- 
transmission,  operated  by  turbines  under  a  moderate  head, 
the  Folsom,  Cal.,  installation  shown  in  Plate  XI  is  worth 
studying.  Fig.  i  shows  the  general  character  of  the  power 
house  and  its  relation  to  the  forebay,  penstocks,  and  tail  race. 
The  forebay  itself  is  double,  being  divided  lengthwise  by  a 
wall,  on  each  side  of  which  are  the  gates  and  penstocks  for 
two  double  turbines.  The  tail  races  are  four  masonry  arches 
under  the  power  house,  uniting  then  into  a  single  channel. 
The  tubular  steel  penstocks  are  8  feet  in  diameter,  and 
the  relief  pipes  above  them,  4  feet  in  diameter.  The  gates 
are  handled  by  hydraulic  cylinders,  like  the  head  gates  at  the 
dam.  It  will  be  observed  that  the  wheel  pit  is  not  in  the 
power  house,  but  in  the  clear  space  between  the  rear  wall  of 


PLATE   XI. 


THE   ORGANIZATION  OF  A    POWER   STATION.        4° I 

the  power  house  and  the  end  wall  of  the  forebay,  which 
like  the  other  masonry  work  in  this  plant  is  of  granite  blocks. 
The  power  house  itself  is  a  spacious  two-story  brick  structure 
on  granite  foundations.  The  lower  floor  is  the  dynamo  room 
while  the  upper  floor  contains  the  transformer  room,  storage 
space,  and  so  forth,  together  with  the  high  tension  switch- 
board, the  lines  from  which  are  shown  running  out  from  the 
end  of  the  building.  The  wheels  are  30"  double  horizontal 
turbines  of  the  McCormick  type,  giving  about  1,250  HP  per 
pair  at  300  revolutions  per  minute  under  the  available  normal 
head  of  55  feet.  There  are,  besides,  two  small  single  horizon- 
tal wheels  for  driving  the  exciters.  Each  of  the  main  wheel 
units  carries  on  its  shaft  a  15,000  Ib.  flywheel  to  steady  its 
operation  under  varying  loads. 

The  arrangement  of  the  wheels  and  generators  is  admirably 
shown  in  Fig.  2,  Plate  XI,  from  a  photograph  taken  during 
the  process  of  construction. 

This  gives  a  view  of  one  complete  unit:  generator,  coupling, 
governor,  turbines  and  flywheel,  and  includes  also  an  exciter 
and  its  wheel,  not  yet  aligned  and  coupled.  Four  such  main 
units  and  the  two  exciters,  all  placed  side  by  side  in  a  single 
row,  make  up  the  plant. 

The  generators  are  three-phase  machines,  of  750  KW 
capacity,  at  60  ~.  Each  has  24  poles,  runs  at  300  revolutions 
per  minute,  and  weighs  about  30  tons.  They  are  of  very  low 
inductance,  with  polyodontal  bar-wound  armatures  designed  to 
give  normally  800  volts  between  lines,  and  to  produce  a  very 
close  approximation  to  a  true  sinusoidal  wave  form.  They 
are  normally  intended  to  run  in  parallel,  although  there  is 
actually  a  complete  circuit  per  machine  available  when  wanted. 
The  wheels  were  originally  installed  with  Faesch-Piccard 
governors,  which  functioned  fairly  well  but  were  not  strong 
enough  for  the  heavy  service,  and  have  now  been  replaced. 

When  the  heavy  apparatus  was  all  in  place  and  connected, 
the  arched  spaces  shown  in  Fig.  2  were  walled  up  except  for 
shaft  holes,  and  the  wheel  pit  permanently  separated  from 
the  dynamo  room.  From  the  dynamos  the  current  is  taken 
to  the  low  tension  switchboard  facing  the  row  of  generators. 
Thence  it  passes  to  the  transformer  room  on  the  second  floor 
of  the  station.  Here  is  a  bank  of  twelve  raising  transformers 


402  ELECTRIC    TRANSMISSION  OF  POWER 

of  the  air  blast  substation  type  largely  used  in  ^he  practice  of 
the  General  Electric  Company.  These  raise  the  working  press- 
ure to  n.ooo  volts.  At  this  potential  the  current  passes  to  the 
high  tension  sv»  itchboard  and  thence  to  the  line.  A  second 
switchboard  in  the  transformer  room  serves  to  distribute  the 
low  tension  current  received  from  the  dynamos. 

The  line  consists  of  four  complete  three-phase  circuits  each 
of  No.  o  B.  &  S.  wire.  There  are  two  independent  pole  lines 
running  side  by  side  a  few  rods  apart,  constructed  of  red- 
wood poles  40  feet  long.  Each  pole  line  carries  two  circuits 
symmetrically  arranged  on  two  cross  arms,  one  circuit  being 
on  each  side  of  the  pole,  the  wires  arranged  so  as  to  form  an 
equilateral  triangle,  with  an  angle  downward.  One  of  the 
pole  lines  carries  an  extra  cross  arm  a  few  feet  below  the 
main  circuits,  to  accommodate  the  telephone  circuit.  All  wires 
are  transposed  at  frequent  intervals  to  lessen  induction.  The 
pole  line  is  on  the  southern  side  of  the  American  River  and 
follows  in  the  main  the  county  roads  clear  into  Sacramento, 
the  two  lines  being  on  opposite  sides  of  the  road.  The  route 
thus  followed  is  a  trifle  longer  than  the  actual  linear  distance, 
but  the  gain  in  accessibility  more  than  counterbalances  the 
extra  mile  or  so  of  line.  The  high  tension  line  is  carried 
along  the  river  through  the  northern  edge  of  the  city  fairly 
into  the  district  of  load,  and  is  then  terminated  in  a  handsome 
brick  substation  containing  the  transformer  and  dynamo 
rooms  and  the  offices  of  the  company.  The  distribution  system 
is  mixed  in  character  owing  to  the  operation  of  the  existing 
railway  and  lighting  loads.  The  dynamo  room  contains  three 
300  HP  three-phase  synchronous  motors  coupled  to  a  line 
shaft,  from  which  are  driven  the  railway  generators  and  the 
arc  machines  for  the  city  and  commercial  circuits.  These 
motors  receive  current  at  500  volts  from  the  reducing 
transformers  in  the  second  story. 

The  main  distribution  circuit  is  a  three-phase  four-wire 
circuit  worked  at  125  volts  between  the  active  wires  and  the 
neutral.  This  gives  an  admirable  network  for  lighting  and 
motor  work,  very  economical  of  copper,  easy  to  wire  and  to 
operate.  Al'  the  transformers  in  the  substation  are  arranged 
for  a  secondary  voltage  of  125,  250,  or  500  as  may  be  desired^ 
so  as  to  be  ready  for  any  kind  of  service. 


THE   ORGANIZATION  OF  A    POWER   STATION 

This  plant  first  went  into  operation  in  July,  1895,  and  nas 
since  then  been  in  continuous  service  day  and  night.  No  seri- 
ous trouble  has  been  encountered,  the  high  voltage  line  has 
performed  admirably,  and  there  has  been  no  difficulty  due  to 
inductance,  lack  of  balance,  resonance,  or  any  of  the  other 
things  that  used  to  be  feared  in  connection  with  long  distance 
polyphase  work  Furthermore  the  plant  is  a  success  financially 
as  well  as  electrically  Apart  from  Niagara,  which  even  now 
is  only  beginning  long  distance  work,  it  is  one  of  the  most 
striking  and  typical  transmission  plants  yet  installed  in  this, 
country. 

Another  fine  example  of  three-phase  work,  of  especial  interest 
as  being  operated  under  the  highest  head  yet  attempted  on 
so  large  a  scale,  is  the  plant  utilized  at  Fresno,  Cal.  Fresno  is 
a  flourishing  city  of  15,000  inhabitants  at  tne  head  01  tne 
magnificent  San  Joaquin  valley  in  central  California.  Like 
other  Californian  cities  it  has  been  hampered  in  its  development 
by  the  very  high  cost  of  coal — $8  to  $10  per  ton  in  carload  lots, 
and  some  of  its  active  citizens  cast  about  for  an  available 
water  power  to  develop  electrically.  Such  an  one  was  found  on 
the  north  fork  of  the  San  Joaquin  River  very  nearly  35  miles 
from  the  city.  At  a  point  where  this  stream  flows  through  a 
narrow  canon  it  was  diverted,  and  the  stream  was  carried  in  a 
series  of  flumes  and  canals  winding  along  the  hillsides  for 
seven  miles  to  a  point  where  it  could  be  dropped  back  into  the 
, river  bed  1,600  feet  below. 

At  this  point  an  emergency  reservoir  was  formed  in  a 
natural  basin  which  by  an  expenditure  of  less  than  $3,000  was 
developed  into  a  pond  capable  of  holding  enough  reserve 
water  for  several  days'  run  at  full  load. 

The  minimum  flow  of  the  stream  is  3,000  cubic  feet  per 
minute,  capable  of  giving  between  6,000  and  7,000  HP  off  the 
shafts  of  the  water  wheels  when  fully  utilized.  In  the  initial 
plant  only  a  small  portion  of  this  power  is  employed.  From 
the  head  works  at  the  reservoir  a  pipe  line  is  taken  down  the 
hillside  to  the  power  house.  The  pipe  is  4,100  feet  long.  At 
the  upper  end  for  400  feet  a  24  inch  riveted  steel  pipe  is  used, 
then  lap-welded  steel  pipe  is  employed  diminishing  in  diameter 
and  increasing  in  thickness  toward  the  lower  end,  where  it 
is  18  inches  in  diameter,  of  five-eighths  inch  mild  steel,  and 


404  ELECTRIC    TRANSMISSION  OF  POWER. 

terminating  in  a  tubuiar  receiver  30  inches  in  diameter,  of 
three-fourths  inch  steel.  The  vertical  head  is  1,410  feet.  This 
corresponds  to  a  pressure  of  613  Ibs.  per  square  inch,  while  the 
emergent  jet  has  a  spouting  velocity  of  300  feet  per  second 

To  withstand  and  utilize  this  tremendous  velocity  unusual  pre- 
cautions were  necessary.  The  main  Pelton  wheels,  designed 
for  500  HP  at  600  revolutions  per  minute,  have  solid  steel  plate 
centres  with  hard  bronze  buckets.  Each  carries  on  its  shaft  a 
steel  flywheel  weighing  3  tons  and  5  feet  in  diameter.  With 
their  enormous  peripheral  speed  of  over  9,000  feet  per  minute, 
these  have  a  powerful  steadying  effect  on  the  speed  of  the 
generators.  There  are  three  of  these  wheels,  each  directly 
coupled  to  a  350  KW  General  Electric  three-phase  generator, 
giving  700  volts  at  6o~.  There  are  also  two  20  HP  Pelton 
wheels,  each  20  inches  in  diameter,  and  each  direct  coupled  to 
a  multipolar  exciter.  All  the  wheels  are  controlled  independ- 
ently by  Pelton  differential  governors. 

On  the  main  floor  of  the  power  house  opposite  the  generators 
is  the  bank  of  raising  transformers.  These  are  six  in  number, 
of  125  KW  capacity  each,  of  the  ordinary  air  blast  type. 
Space  is  provided  for  three  more  when  the  load  demands 
them. 

These  transformers  raise  the  pressure  to  11,200  volts 
between  lines  and  from  the  high  tension  section  of  the  switch- 
board the  current  passes  to  the  transmission  line.  This  con- 
sists of  two  complete  three-phase  circuits  which  can  be  worked 
together  or  independently.  They  are  of  No.  oo  bare  copper 
wire  carried  on  special  double  petticoat  porcelain  insulators, 
all  tested  at  27,000  volts  alternating  pressure. 

The  pole  line  is  of  35  foot  squared  redwood  poles  set  6  feet 
deep.  Each  pole  carries  four  cross  arms.  Three  of  these  at 
the  top  of  the  pole  are  for  the  transmission  circuits.  These 
are  at  present  confined  to  the  two  upper  cross  arms,  leaving 
space  for  additional  circuits  below.  A  fourth  short  cross  arm 
about  4  feet  below  the  others  carries  the  telephone  wires. 

Plate  XII  gives  a  good  idea  of  the  general  arrangement  of 
the  Fresno  plant.  Fig.  i  gives  a  glimpse  of  the  storage 
reservoir  at  the  upper  end  of  the  pipe  line.  Fig.  2  shows  the 
situation  of  the  power  house  below,  which  is  built  of  native 
granite  on  a  solid  rock  foundation,  with  a  wooden  roof.  It  is 


PLATE   XIII. 


THE    ORGANIZATION  OF  A    POWER   STATION.        405 

75  X  3°  feet  in  size.  The  wheel. pit  is  seen  running  along  one 
side  of  the  station  just  outside  the  wall,  through  which  pass  the 
wheel  shafts  driving  the  dynamos  inside.  In  the  foreground 
appears  the  beginning  of  the  transmission  line. 

Fig.  3  shows  the  interior  of  the  power  house  with  the  dyna- 
mos and  transformers  in  place  and  the  switchboard  at  the 
further  end  of  the  room. 

In  the  city  of  Fresno  the  transmission  lines  are  taken  to  a 
substantial  brick  substation  in  the  centre  of  the  city.  Here 
are  situated  the  reducing  transformers  and  accessory  apparatus, 
including  two  So-light  arc  dynamos  direct  coupled  to  60  HP 
induction  motors. 

The  distribution  system  is  threefold.  In  the  central  dis- 
trict of  the  city  a  three-phase  four-wire  network  is  employed, 
supplied  from  three  125  KW  reducing  transformers,  and 
worked  at  115  volts  between  active  wires  and  neutral.  For 
the  outlying  residence  region  three  75  KW  transformers  supply 
current  at  1,000  volts  for  use  with  secondary  transformers. 
Finally,  for  reaching  neighboring  towns,  three  40  KW  trans- 
formers feed  a  3,000  volt  sub-transmission  system.  The  oper- 
ation of  this  plant,  like  that  of  the  Folsom  plant,  has  been 
highly  successful  from  the  start,  and  the  electrical  troubles 
that  have  often  been  feared  on  long  lines  at  high  voltage  have 
been  conspicuous  by  their  abrence. 

Both  these  plants  represent  the  best  modern  practice  in 
general  equipment  and  arrangement,  and  while  differing  con- 
ditions bring  their  own  necessary  modifications,  these  examples 
may  be  regarded  as  thoroughly  typical.  They  have  incident- 
ally demonstrated  the  thorough  practicability  of  general  dis- 
tribution of  energy  for  lighting  and  power  by  polyphase  cur- 
rente  under  large  commercial  conditions,  and  at  distances 
great  enough  to  involve  all  the  electrical  difficulties  likely 
to  be  met  at  the  voltage  employed.  A  more  recent  plant  of 
peculiar  interest  in  some  of  its  engineering  features  is  that  of 
the  Truckee  River  General  Electric  Company  near  Floriston, 
Cal.,  shown  in  Plates  XIII  and  XIV.  This  plant  was  erected 
to  supply  power  to  the  mines  of  the  famous  Comstock 
Lode,  where  it  is  used  for  mining  hoists,  milling  and  pumping, 
which  had  formerly  been  done  almost  entirely  by  steam  pro- 
vided by  burning  pine  at  $8.50  to  $15  per  cord. 


406  ELECTRIC    TRANSMISSION  OF  POWER. 

The  source  of  the  water  power  is  the  Truckee  River,  an 
unusually  steady  stream  rising  among  the  snows  of  the  Sierra- 
Nevada.  At  the  head  works  is  a  timber  crib  dam  about  50  yards 
long  and  only  7  feet  in  height,  serving  mainly  to  back  the 
water  into  a  wide,  slow  running  canal  a  couple  of  hundred  yards 
long,  which  serves  also  as  a  settling  pond.  Thence  the  water 
passes  through  the  racks  into  a  timber  flume,  10'  deep  and 
6'S"  wide  inside,  the  entrance  being  widened  to  28'  at  the  racks 
and  tapered  to  the  normal  width  in  a  run  of  40'. 

This  timber  flume,  a  portion  of  which  is  well  shown  in  Plate 
XIII,  Fig.  i,  winds  along  the  hillsides  for  a  distance  of  a  little 
more  than  a  mile  and  a  half.  It  carries  300  cubic  feet  of  water 
per  second  at  a  depth  of  6' in  the  flume,  the  corresponding 
velocity  being  7.5  feet  per  second.  This  flume  is  carried  on 
heavy  timber  frames  16'  between  centres,  with  two  inter- 
mediate sets  of  four  posts  each.  Along  the  line  of  the  flume 
are  two  spill  gates  each  in  the  side  of  a  sand  box  dropped 
below  the  bottom  of  the  flume. 

This  flume  terminates  in  a  timber  penstock  36'  long  and  21' 
wide  furnished  with  a  central  bulkhead  and  strongly  stayed 
with  iron  rods.  Back  of  the  penstock  a  spill  flume  is  carried 
for  200'  alongside  the  main  flume.  From  the  penstock  two- 
pipes,  takingtheir  water  through  head  gates,  run  to  the  wheels. 

These  pipes  are  of  redwood  staves,  hooped  with  |"  round 
steel,  6'  in  diameter  inside,  and  160'  long.  The  working  head 
is  84.5  feet,  and  a  few  feet  from  the  power  house  the  wooden 
pipes  are  wedged  into  the  steel  pipes  that  lead  to  the  wheel 
cases.  Plate  XIII,  Fig.  2,  shows  the  power  house,  penstock, 
pipes  and  tailraces.  The  power  house  itself  is  88'  x  31',  of 
brick,  with  roof  of  corrugated  galvanized  iron,  and  has  con- 
crete foundations. 

Plate  XIV  shows  the  arrangement  of  the  wheels  and  gener- 
ators. The  wheel  plant  consists  of  two  pairs  of  27"  McCor- 
mick  horizontal  turbines,  each  pair  giving  1,400  HP  at  400 
r.p.  m.  Each  pair  discharges  into  a  central  cast  iron  draught 
box  continued  by  a  20  draught  tube.  Each  pair  of  wheels  is  di- 
rectly coupled  to  a  750  KW,  500  volt  three-phase  Westinghouse 
generator.  But  instead  of  the  arrangement  shown  in  Plates 
XI  and  XII  the  shafts  of  both  sets  of  wheels  and  of  the  eener- 

o 

ators  are  in  one  straight  line,  with  the  wheels  at  its  extremities 


PLATE   XIV. 


THE    ORGANIZATION    OF  A    POWER    STATION.         40? 

This  gives  space  for  a  very  solid  foundation  for  the  generators 
between  the  arched  tailraces,  and  if  need  be  the  generators 
can  be  directly  coupled  together  so  as  to  run  both  from  a 
single  wheel  or  as  a  single  unit.  Each  generator  has  a 
separate  multipolar  exciter  driven  by  a  small  separate  turbiae, 
and  each  of  these  receives  the  water  from  the  case  of  its  main 
wheel  and  discharges  into  the  corresponding  tailrace.  Each 
exciter  is  of  sufficient  capacity  for  both  generators. 

Each  main  pair  of  wheels  is  regulated  by  a  Lombard  gov- 
ernor, one  of  which  appears  in  the  foreground.  But  to  en- 
sure close  regulation  an  unusual  device  is  installed  in  connec- 
tion with  the  governors.  The  supply  pipes  are  too  long  and 
the  head  too  high  to  permit  the  installation  of  efficient  relief 
pipes,  as  in  the  Folsom  plant,  and  the  enormous  inertia  of  the 
water  in  the  supply  pipes  was  consequently  both  an  incon- 
venience and  a  menace.  Hence  relief  was  provided  by  a 
huge  balanced  Ludlow  valve  connected  with  the  wheel  case  and 
the  tailrace.  This  valve  is  operated  by  wire  ropes  and  sheaves 
so  connected  with  the  gate  shaft  of  the  wheel  that  when  the 
governor  closes  the  wheel  gate  it  opens  the  relief  valve  and 
vice  versa,  thus  keeping  the  velocity  of  the  water  nearly  con- 
stant. The  effect  is  closely  similar  to  that  obtained  with  the 
deflecting  nozzle  used  with  Pelton  wheels,  and  while  it  wastes 
water,  that  is  of  small  moment  compared  with  the  necessity 
for  regulation. 

The  500  volt  current  from  the  generators  is  raised  by  oil 
insulated  transformers  to  22,000  volts  for  the  33-mile  trans- 
mission to  Virginia  City,  Nev._,  which  is  the  centre  of  utili- 
zation. The  pole  line  is  of  square  sawed  redwood  poles  30' 
long,  n"  square  at  the  butt  and  7"  square  at  the  top.  These 
poles  carry  two  cross  arms  on  which  the  two  three-phase  cir- 
cuits of  bare  No.  4  B.  &  S.  wire  are  arranged  as  usual,  forming 
an  equilateral  triangle  on  each  side  of  the  poles.  The  insula- 
tors are  porcelain  on  oil-treated  eucalyptus  pins.  The  poles 
are  spaced  about  40  to  the  mile,  and  carry  a  couple  of  brackets 
for  the  telephone  line  below  the  cross  arms.  The  three-phase 
lines  are  transposed  every  144  poles. 

The  distribution  in  Virginia  City  is  at  2,250  volts  three- 
phase  over  a  maximum  radius  of  about  2  miles.  This  plant  is 
a  good  example  of  recent  practice  in  dealing  with  moderately 


408  ELECTRIC   TRANSMISSION  OF  POWER. 

high  heads.  The  timber  flume  in  particular  strikes  Eastern 
engineers  unfavorably  at  first,  but  the  irrigation  companies  of 
the  Pacific  slope  have  had  many  years  of  experience  in  that 
sort  of  construction,  and  have  learned  that  it  is  easy,  cheap, 
and  durable  when  properly  cared  for.  There  are  hundreds  of 
miles  of  it  used  for  various  purposes  in  California,  and  in 
many  instances  it  is  the  only  practicable  means  of  water 
delivery.  Altogether  this  particular  plant  teaches  a  useful 
lesson  in  hydraulic  construction,  and  like  those  just  described 
is  a  very  good  example  of  modern  engineering. 

At  present  long  distance  plants  are  rather  the  exception,  and 
in  the  natural  course  of  events  there  must  be  developed  a  great 
number  of  power  transmissions  at  quite  moderate  distances, 
under  ten  miles  or  so.  Such  plants  as  regards  general  organi- 
zation do  not  possess  any  special  peculiarities.  The  dynamos, 
however,  may  often  be  wound  for  exceptionally  high  voltage. 
Dynamos  for  use  with  raising  transformers  should  be  of 
moderate  voltage,  seldom  over  1,000  volts  unless  the  units  are 
of  immense  size,  or  must  furnish  local  power  in  addition  to 
their  regular  function. 

At  moderate  voltage  the  generators  gain  in  cost  per  unit  of 
output,  in  simplicity,  and  in  comparative  immunity  from  acci- 
dents. They  are  also  likely  to  be  designed  for  lower  arma- 
ture reaction.  Nevertheless,  there  are  many  cases  in  which 
generators  for  5,000  to  12,000  volts  may  be  properly  employed 
for  the  sake  of  economy  and  simplicity  of  plant.  As  already 
indicated  such  generators  should  preferably  have  stationary 
armatures,  and  should  have  extraordinarily  good  insulation. 
When  installed  they  should  be  insulated  from  the  foundations 
with  scrupulous  care,  and  when  direct-coupled  should  be 
provided  with  insulating  couplings.  Small  high-voltage 
machines  are  sometimes  supported  on  porcelain  insulators. 
Large  generators  may  be  carried  on  hardwood  timbers  thor- 
oughly treated  with  insulating  material,  and  bolted  to  the 
foundation  cap  stone;  or,  at  higher  voltages,  on  polished  dense 
stone  blocks  set  in  sulphur  and  surrounded  with  oil  grooves 
like  those  on  a  high  voltage  insulator,  and  in  divers  kindred 
ways  which  will  suggest  themselves  to  the  constructor.  It  is 
desirable  to  surround  such  machines  with  an  insulated  plat- 
form a  few  inches  above  the  floor,  and  to  protect  the  leads 


THE   ORGANIZATION  OF  A    POWER   STATION.        409 

with  vulcanite  tubes.  It  is  well  also  to  shield  the  terminals  so 
that  only  one  can  be  manipulated  at  a  time  when  the  machine 
is  in  action. 

In  all  plants  employing  more  than  a  single  generator,  and 
this  means  nearly  all  power  transmission  plants  of  every  kind, 
the  generators  should  be  arranged  to  run  in  parallel,  and  in 
most  instances  should  be  so  operated  regularly.  Now  and 
then  generators  may  advantageously  be  operated  on  separate 
lines,  as  when  these  lines  must  be  run  under  different  condi- 
tions of  regulation,  cr  when  a  line  must  be  isolated  for  the 
purpose  of  carrying  a  very  severe  fluctuating  load,  but  for  the 
vast  majority  of  plants  these  expedients  are  totally  unneces- 
sary, and  only  complicate  the  operation  of  the  system  without 
any  material  compensating  advantage. 

Plants  operated  for  lighting  alone  can  get  along  after  a 
fashion  by  shifting  load  quickly  from  one  machine  to  another, 
an  operation  quite  familiar  to  most  people  who  have  been  cus- 
tomers of  such  a  system;  but  for  the  general  distribution  of 
lights  and  power  this  procedure  is  inadmissable,  for  it  usually 
means  stopping  some  or  all  of  the  motors.  Moreover  it  is  a 
clumsy  method  at  best,  abandoned  long  ago  by  continuous 
current  stations,  and  without  any  excuse  for  existence  save 
villainously  bad  generator  equipment  or  incompetence  in  the 
operation  of  the  station. 

All  modern  generators  of  good  design  are  capable  of  running 
in  parallel  without  the  slightest  difficulty,  provided  they  have 
somewhere  nearly  similar  magnetic  characteristics  and  are 
intelligently  operated. 

It  is  inadvisable  to  attempt  running  a  smooth-core  and  an 
iron-clad  armature  in  parallel,  or  two  machines  which  are  very 
different  in  regulation  or  which  give  very  different  wave  shape, 
but  on  the  other  hand  such  machines  ought  not  to  be  installed 
together  on  general  principles.  The  nearer  alike  the  machines 
the  better  they  will  run  in  parallel. 

No  subject  has  been  oftener  a  topic  of  fruitless  discussion 
than  the  paralleling  of  alternators.  As  a  matter  of  fact  any 
two  similar  alternators  will  go  into  parallel  and  stay  there  with 
very  little  difficulty,  at  least  if  driven  from  water  wheels,  as  is 
nearly  always  the  case  in  transmission  plants. 

High  inductance  machines  have  been  supposed  to  be  some- 


410  ELECTRIC  TRANSMISSION  OF  POWER, 

what  easier  to  put  and  work  in  parallel  than  those  of  low 
inductance.  They  certainly  can  be  thrown  together  carelessly 
with  less  likelihood  of  a  large  synchronizing  current  flowing 
between  them,  but  with  low  inductance  machines  a  little  more 
care,  or  an  inductance  temporarily  inserted  between  the 
machines,  leads  to  the  same  end. 

In  throwing  two  alternators  of  any  kind  in  parallel,  they 
should  be  in  the  same  phase,  running  at  the  same  speed  and  at 
approximately  the  same  voltage.  The  more  nearly  these  con- 
ditions are  fulfilled  the  less  synchronizing  current  will  flow 
between  the  machines,  and  hence  the  more  smoothly  will  they 
drop  together. 

The  ordinary  arrangement  of  phase  lamps  shows  the  relation 
of  both  speed  and  phase  with  ample  exactness.  When  the 
indicator  lamp  is  pulsating  at  the  rate  of  one  period  in  four  or 
five  seconds,  it  is  evident  that  the  relative  speeds  of  the 
machines  are  very  nearly  right,  and  it  is  quite  easy  to  cut  in 
the  new  machine  when  its  phase  is  very  nearly  right.  One 
soon  gets  the  swing  of  the  slow  pulsations,  and  can  catch  the 
middle  point  of  the  interval  of  darkness  with  great  accuracy. 
The  pulsations  can  in  fact  be  easily  reduced  to  a  ten-second 
period  or  even  longer.  It  is  on  the  whole  best  to  reverse  the 
phase  lamp  connections  so  that  concordance  of  phase  will  be 
marked  by  the  lighting  up  of  the  phase  lamps.  The  lamps 
should  be  of  such  voltage  that  they  will  come  merely  to  a 
bright  red  when  the  machines  are  in  phase.  This  arrange- 
ment averts  the  possibility  of  a  lamp  burning  out  during 
phasing  and  giving  apparent  concordance  of  phase.  This 
accident  has  actually  happened — with  spectacular  results. 

It  is  obviously  necessary  that  the  speeds  of  the  two  machines 
should  be  normally  alike,  and  that  the  speeds  should  have  a 
certain  slight  flexibility.  When  belt  driven  from  the  same 
shaft  the  various  generators  to  be  put  in  parallel  must  be  run 
.very  accurately  at  the  same  speed,  else  one  of  the  belts  will 
constantly  slip  and  there  will  be  considerable  synchronizing 
current.  When  properly  adjusted  the  machines  should  be  so 
closely  at  speed  that  the  phase  lamps  will  have  a  period  of 
from  20  to  30  seconds.  This  is  not  a  difficult  matter  when 
driving  from  the  same  shaft.  In  direct-coupled  units,  or  in 
general  those  driven  from  independent  prime  movers,  it  is  best 


THE   ORGANIZATION   OF  A    POWER   STATION.        4lr 

to  let  one  governor  do  the  fine  adjustment  of  speed,  the  others 
being  a  little  more  insensitive.  Otherwise  the  governors  are 
likely  to  fight  among  themselves  and  be  perpetually  see-sawing. 

With  respect  to  equality  of  voltage,  the  better  the  regula- 
tion of  the  generators  in  themselves  the  more  necessary  it  is  ta 
have  them  closely  at  the  same  voltage  when  put  into,  or  when 
running  in,  parallel.  Two  generators  with  bad  inherent  regu- 
lation will  divide  the  load  with  approximate  equality,  even  if 
put  in  parallel  with  a  noticeable  difference  in  voltage,  since  the 
machine  that  tends  to  take  the  heavier  current  will  promptly 
have  its  voltage  battered  down  and  the  tendency  corrected — 
at  the  expense,  however,  of  accurate  regulation  in  the  plant. 

With  machines  of  low  inductance  and  good  regulation,  the 
voltages  should  be  very  closely  the  same  before  putting  into 
parallel,  to  avoid  a  heavy  synchronizing  current,  and  they  wilt 
then  divide  the  load  correctly  with  a  very  slight  adjustment 
of  the  voltage.  If  the  characteristics  of  the  machines  are 
known,  as  they  should  be,  the  voltages  can  be  arranged  so 
that  they  will  fall  together  as  accurately  as  if  the  added 
machine  had  been  put  on  an  artificial  load  before  parallelizing. 

If  these  precautions  are  observed  no  difficulty  will  be 
experienced  in  parallel  running,  and  machines  in  stations 
many  miles  apart  will  work  together  in  perfect  harmony. 
This  is  sometimes  necessary  in  large  central  station  work, 
when  a  portion  of  the  power  is  transmitted  from  a  distance 
and  a  portion  generated  on  the  spot.  It  sometimes  happens, 
too,  that  to  obtain  the  amount  of  water  power  that  is  desired 
it  must  be  taken  from  a  group  of  falls. 

The  magnitude  of  the  transformer  units,  when  transformers 
are  used,  should  be  determined  by  the  same  considerations  that 
apply  to  generators,  except  that  questions  of  speed  do  not 
have  to  be  considered.  The  smallest  number  of  transformers 
that  it  is  desirable  to  use  is  that  number  which  will  permit 
the  disuse  of  a  single  unit  without  inconvenience.  Above  this 
number  one  must  be  guided  by  convenience,  but  in  general 
the  fewer  units  the  better,  since  transformers  such  as  are 
used  in  large  transmission  work  vary  very  little  in  efficiency 
under  varying  load,  and  hence  there  is  no  considerable  gain 
in  using  small  units  so  as  to  keep  them  fully  loaded.  When 
using  large  transformers  the  difference  in  efficiency  between 


412  ELECTRIC    TRANSMISSION  OF  POWER. 

full  load  and  half  load  should  be  no  more  than  two  or  three- 
tenths  of  a  per  cent.,  and  as  a  rule  the  general  efficiency 
cannot  be  sensibly  improved  by  using  smaller  units. 

In  polyphase  transmission  the  transformer  unit  must  be 
taken  to  include  all  the  phases,  so  that  this  unit  will  usually  con- 
sist of  two  or  three  allied  transformers.  In  three-phase  work 
the  circuit  can  be  operated  either  with  two  or  three  trans- 
formers, so  that  in  a  measure  each  transformer  group  contains 
a  reserve  of  capacity,  since  if  a  transformer  fails  the  remain- 
ing pair  can  be  connected  to  do  nearly  two-thirds  of  the  work. 
It  is  inadvisable,  however,  to  try  the  resultant  mesh  on  a  large 
scale  save  as  an  emergency  expedient,  and  the  raising  and 
reducing  transformers  should  regularly  be  in  groups  of  three  for 
three-phase  work,  connected  star  or  mesh  as  occasion  requires. 
For  very  high  voltage  each  phase  may  have  several  transformers 
in  series. 

It  is  advisable  in  arranging  the  transformer  plant  to  bear 
this  in  mind.  A  spare  transformer  or  two  is  a  good  form 
of  insurance.  In  the  station  raising  transformers  alone  are 
concerned.  These  are  likely  to  be  of  large  capacity  and  high 
voltage.  The  individual  transformers  will  very  seldom  be  as 
small  as  50  KW,  and  the  voltage  is  sure  to  be  from  5,000  volts 
upward  to  10,000,  15,000,  or  20,000  volts,  and  sometimes 
even  more. 

Such  transformers  should  be  treated  with  the  respect  that 
their  voltage  demands.  It  is  always  best  to  install  them  in  a 
separate  room  or  otherwise  to  isolate  them  so  that  they  shall  be 
accessible  only  to  those  persons  directly  concerned  with  them. 

They  should  be  very  thoroughly  insulated  from  the  ground. 
As  good  a  way  as  any  is  to  carry  them  on  porcelain  blocks 
upon  a  floor  of  dry  wood  covered  or  saturated  with  insulating 
compound.  A  good  precaution  is  to  carry  around  them  a 
floor  of  treated  wood  supported  on  porcelain  insulators  and 
of  such  width  that  anyone  touching  the  transformers  must 
have  stepped  upon  the  insulated  platform.  The  transformers 
should  not  be  crowded  and  the  high  voltage  leads  should  be 
taken  out  in  plain  sight  and  carried  on  porcelain  insulators. 
Each  transformer  should  have  independent  means  for  cutting 
it  into  and  out  of  service.  Such  switches  should  be  insulated 
with  extraordinary  care.  It  is  best  to  have  no  exposed  metal 


THE   ORGANIZATION   OF  A    POWER   STATION.         4*3 

on  the  high  tension  switches,  and  long  insulated  handles,  and 
one  should  locate  all  the  high  tension  apparatus  together 
on  a  special  switchboard  guarded  by  an  insulated  platform. 
Switches  working  in  oil  arc  very  satisfactory  for  high  voltage 
work,  but  one  must  guarJ  against  the  temptation  of  making 
them  two  compact.  All  1-ads  to  and  from  this  board  ought  to 
be  run  in  as  straightforward  a  way  as  possible,  so  that  each 
wire  can  be  traced  without  the  slightest  effort,  and  all  the  con- 
nections at  the  back  of  the  board  should  be  simply  arranged 
with  ample  space  between  them.  The  fundamental  rule  for  all 
high  voltage  wiring  should  be  to  keep  the  wires  well  apart 
and  in  plain  view  from  the  generators  or  transformers  to 
their  exit  from  the  building.  Finally,  the  transformers  and 
all  their  connections  should  always  be  treated  as  dangerous  in 
spite  of  all  precautions. 

These  directions  may  seem  at  first  sight  over-cautious,  but 
it  must  be  remembered,  first,  that  the  raising  transformers 
and  their  connections  carry  the  highest  voltage  that  conditions 
permit,  and  second,  that  such  voltage  is  highly  formidable; 
10,000  volts  even,  now  to  be  regarded  as  an  ordinary  voltage 
for  transmission  work,  has  little  respect  for  anything  but 
extraordinary  insulation  and  plenty  of  it,  while  at  higher 
voltages  the  striking  distance  rapidly  increases.  The  static 
effects  of  these  pressures  are  prodigious,  and  neighboring 
wires,  even  if  disconnected,  must  be  handled  with  caution. 

In  small  stations  it  maybe  convenient  to  install  all  the  appa- 
ratus in  a  single  room,  but  even  then  the  transformer  plant 
should  be  carefully  guarded. 

In  many  cases  large  transformers  are  artificially  cooled  and 
must  be  installed  with  that  in  view,  but  air  blasts  or  circulating 
pumps  do  not  involve  any  special  considerations  except  that 
spare  parts  must  be  at  hand  in  case  of  accident. 

It  should  of  course  be  understood  that  transformers  in  power 
transmission  work  can  be,  and  very  often  are,  worked  in  par- 
allel with  the  greatest  facility.  Transformers  to  be  so  used 
must  have  closely  similar  magnetic  characteristics,  and  par- 
ticularly must  regulate  alike  under  varying  loads.  They  must 
also  have  independent  fuses  or  other  safety  devices,  so  that 
each  can  take  care  of  itself.  In  all  cases  it  is  highly 
desirable  to  have  one  or  more  spare  transformers  ready 


414  ELECTRIC   TRANSMISSION  OF  POWER. 

to  be  cut  in  at  a  moment's  notice  anywhere  that  may  be 
necessary. 

Where  transportation  is  difficult  the  installation  of  trans- 
formers is  rather  a  serious  problem.  Generally  speaking  it  is 
best  to  sectionalize  the  coils,  each  section  being  independent 
and  fully  insulated.  The  core  plates  can  then  be  taken  in  in 
bundles  and  the  transformers  built  up  on  the  spot,  with  what- 
ever additional  insulation  may  be  necessary.  Of  course 
means  must  be  at  hand  for  the  final  testing,  including  a  small 
testing  transformer  to  obtain  the  necessary  voltage. 

The  most  important  accessories  of  a  plant  pertain  to  the 
switchboard.  This  must  be  built  with  particular  care  in  high 
voltage  work.  Its  location,  as  a  general  thing,  should  be  on 
the  floor  of  the  dynamo  room,  except  as  the  high  tension 
section  may  be  placed  in  the  transformer  room  near  the 
apparatus  which  it  serves.  The  general  rule  should  be  so  to 
place  the  switchboard  with  respect  to  its  apparatus  that  it 
shall  be  very  accessible,  and  where  both  may  be  easily  watched 
at  once  by  the  attendants.  Sometimes  in  very  large  stations 
the  switchboard  is  isolated  and  in  constant  charge  of  a  special 
attendant,  but  the  practice  is  not  to  be  generally  recom- 
mended. 

As  to  materials,  marble  is  by  far  the  best  yet  employed  in 
point  of  beauty  and  fine  insulating  properties.  Slate  has  often 
been  used,  but  is  rather  unreliable  both  as  an  insulator  and 
mechanically.  The  switches  may  either  be  on  individual 
bases  and  bolted  to  the  switchboard  or  mounted  directly  upon 
it  without  separate  bases.  For  large  permanent  work  the 
latter  course  is  preferable.  It  is  now  quite  customary  to 
mount  the  equipment  for  each  individual  generator  in  a  single 
complete  panel  and,  except  for  very  large  work,  this  is  exceed- 
ingly convenient. 

High  voltage  switches  require  special  construction.  When 
the  generator  gives  from  5,ooo  to  12,000  volts,  or  when  these 
or  higher  voltages  are  derived  from  transformers,  the  danger 
of  arcing  across  at  the  switch  is  quite  serious  and  the  switch 
gaps  must  be  corresponding  wide.  They  may  often  be  advan- 
tageously reinforced  by  insulating  barriers.  In  a  plant  using 
raising  transformers  the  switches  should  be,  so  far  as  possible, 
on  the  low  voltage  side,  so  that  need  for  opening  a  switch 
under  pressure  on  the  high  voltage  side  may  be  avoided. 


THE    ORGANIZATION   OF  A    POWER   STATION.        4*5 

Modern  generators  with  small  armature  reaction  and  low 
inductance  produce  ferocious  arcing  when  the  circuit  is  opened 
in  oise  of  a  short  circuit,  for  the  voltage  does  not  drop  much. 
Hence  all  safety  devices  must  be  planned  accordingly.  Plenty 
of  room  is  the  main  point.  Magnetic  cutouts  are  generally 
preferable  to  fuses,  but  when  the  latter  are  used  they  must  be 
thoroughly  protected  against  the  maintenance  of  an  arc.  A 
few  close-fitting  large  asbestos- paper  washers  strung  along 
a  fuse  will  break  almost  any  kind  of  an  arc,  but  the  fuses  in 
different  lines  should  be  far  enough  apart  to  prevent  the  arc 
cutting  over  from  line  to  line  instead  of  trying  to  follow  the 
break.  The  modern  inclosed  fuses,  if  on  a  sufficiently  large 
scale,  work  excellently  on  high  voltages.  In  transformer  plants 
the  safety  devices  are  frequently  put  on  the  low  voltage  side  of 
the  circuit,  and  as  a  rule  the  high  tension  lines  are  let  severely 
alone  when  carrying  current. 

Money  speu«  in  accurate  switchboard  and  testing  instru- 
ments is  well  invested.  Ammeters  and  voltmeters  should  be 
of  the  best  quality  and  accurately  adjusted  when  in  place  on 
the  switchboard.  For  station  use  the  instruments  with  very 
large  dials  are  to  be  recommended  for  their  easy  visibility. 
One  should  not  have  to  smell  of  an  instrument  to  discover  the 
reading.  The  voltage  on  high  tension  lines  is  most  conven- 
iently measured  by  transforming  down,  but  in  large  stations 
it  is  desirable  to  have  direct-reading  testing  instruments.  Up 
to  about  3,000  volts  reliable  voltmeters  can  easily  be  obtained, 
but  much  beyond  that  one  must  use  electrostatic  instruments 
or  rely  solely  on  transformation.  •  . 

The  main  circuits  should  be  equipped  with  integrating 
wattmeters  as  a  final  check  on  the  station  output.  This  pre- 
caution is  particularly  valuable  in  alternating  stations  since, 
in  connection  with  the  other  instruments,  it  gives  the  power 
factor  of  the  plant,  which  should  be  closely  watched. 

A  much  neglected  but  highly  desirable  accessory  in  high 
voltage  work  is  some  means  of  measuring  the  line  insulation 
under  the  stress  of  the  normal  voltage. 

In  the  way  of  mechanical  fittings,  the  first  place  should  be 
given  to  a  travelling  crane,  capacious  enough  to  move  every' 
thing  which  is  likely  to  need  moving  about  the  plant.  Not 
only  is  it  exceedingly  useful  in  installation,  but  it  may  be 


41 6  ELECTRIC    TRANSMISSION   OF  POWER. 

needed  for  repairs,  and  in  such  case  may  save  much  valuable 
time. 

It  is  very  important  to  have  at  least  one  man  about  the 
plant  who  is  a  good  practical  mechanic,  and  to  provide  a  work- 
room and  tool  equipment  enough  to  enable  small  repairs  to 
be  made  on  the  spot.  In  most  cases  material  and  tools  for 
minor  electrical  repairs  are  necessary  and  they  are  always 
desirable,  for  they  make  it  possible  to  forestall  further  repairs 
and  often  will  tide  over  an  emergency,  even  if  outside  help 
has  finally  to  be  called  in.  The  more  isolated  the  station  the 
more  necessary  it  is  to  make  such  provisions,  and  the  more 
spare  parts  must  be  at  hand.  Of  line  material  there  should 
always  be  plenty  in  stock  to  repair  breaks,  and  this  stock 
should  never  be  allowed  to  get  low. 

Finally,  as  regards  attendance,  incompetent  men  are  dear  at 
any  price.  It  pays  to  employ  skilled  men  and  to  make  it 
worth  their  while  to  settle  down  to  permanent  work.  They 
are  valuable  all  the  time,  and  can  be  depended  upon  in  an 
emergency  when  less  competent  ones  would  fail.  In  this  as 
in  other  things  avoid  the  fault  stigmatized  in  the  vernacular 
as  "  saving  at  the  tap  and  spilling  at  the  bung-hole." 


CHAPTER  XII. 

THE    LINE. 

THE  line  is  a  very  important  part  of  a  power  transmission 
system,  for  on  its  integrity  depends  the  continuity  of  service 
without  which  even  the  most  perfect  apparatus  is  commercially 
useless.  In  most  cases  the  customer  who  uses  electrical 
power  neither  knows  the  efficiency  of  his  motor  nor  cares 
much  about  it,  so  long  as  the  machine  goes  steadily  along 
without  the  annoyance  and  expense  of  frequent  repairs.  But 
if  the  service  frequently  fails,  suspending  the  operation  of  all 
his  machinery  while  repairs  are  being  executed,  the  electric 
motor,  so  far  as  he  is  concerned,  is  a  commercial  failure,  and 
a  nuisance  to  boot,  and  no  representations  of  cheap  power 
can  be  of  much  avail  when  a  single  stoppage  may  cause  more 
loss  than  could  be  recompensed  by  free  power  for  a  month. 

Modern  dynamos  and  motors  of  almost  every  class  are 
reasonably  efficient  and  reliable,  so  that  as  a  rule  the  line  is 
the  weakest  portion  of  the  system.  More  particularly  is  this 
the  case  when  the  distance  of  transmission  is  great  and  many 
miles  of  line  must  be  guarded,  inspected,  and  kept  in  perfect 
working  order.  In  such  long  lines  not  only  is  the  actual  labor 
of  maintenance  great  but  the  principal  engineering  difficulties 
will  there  be  encountered.  With  apparatus  of  the  character 
even  now  available,  the  future  of  electrical  power  transmission 
depends  in  very  large  measure  on  the  development  that  takes 
place  in  the  construction,  insulation,  and  maintenance  of  the 
line,  together  with  the  solution  of  certain  electrical  problems 
that  arise  as  the  line  grows  longer.  It  is  therefore  important 
to  go  into  the  matter  very  carefully,  as  regards  not  only  the 
general  arrangements  and  the  electrical  details  of  the  work, 
but  with  respect  to  methods  of  construction. 

We  may  then  with  advantage  divide  our  consideration  of  the 
line  into  three  heads.  First,  the  line  in  its  general  relations 
to  the  plant,  considering  it  merely  as  a  conductor.  Second, 

417 


41 8  ELECTRIC   TRANSMISSION  OF  POWER. 

the  line  as  a  special  problem  in  engineering.  Third,  the  line 
as  a  mechanical  structure.  Of  these  heads  the  first  has  to 
do  with  such  questions  as  the  proper  proportioning  of  the  line 
as  a  part  of  the  system,  its  function  as  a  distributing  con- 
ductor, and  its  bearing  on  the  general  efficiency  of  the  plant 
of  which  it  is  a  part.  Next  come  up  for  examination  the 
electrical  difficulties  that  appear  in  the  line,  and  finally  the 
materials  of  construction  and  the  methods  of  applying 
them. 

One  of  the  first  questions  that  arises  in  designing  a  plant 
for  the  transmission  of  power  is  the  character  and  dimensions 
of  the  conducting  system  in  their  relation  to  the  rest  of  the 
plant.  Efficiency  is  generally  the  first  thing  thought  of — cost 
comes  as  a  gloomy  afterthought;  and  between  these  two  good 
service  is  only  too  frequently  neglected.  In  taking  up  a  trans- 
mission problem  the  layman's  first  query  generally  is,  "How 
much  power  will  be  lost  in  the  line?  "  and  when  the  engineer 
answers,  "  As  much  or  as  little  as  you  please,"  the  subject  of 
line  design  is  opened  up  in  its  broadest  aspect. 

Whenever  an  electrical  current  traverses  a  conductor  there 
is  a  necessary  loss  of  energy  due  to  the  fact  that  all  substances 
have  an  electrical  resistance  which  has  to  be  overcome.  The 
energy  so  lost  is  substantially  all  transformed  into  heat,  which 
goes  to  raising  the  temperature  of  the  conductor,  and  indirectly 
that  of  surrounding  bodies.  The  facts  in  the  case  are  put 
in  their  clearest  and  most  compact  form  by  Ohm's  law, 

2? 
C  =  —„.     This  states  that  the  current  is  numerically  equal  to 

the  electromotive  force  between  the  points  where  the  current 
flows,  divided  by  the  resistance.  Hence,  this  E.  M.  F.  equals 
the  current  multiplied  by  the  resistance  between  these  points. 

E  =  CR. 

This  tells  us  at  once  the  loss  in  E.  M.  F.  between  the  ends 
of  any  line  provided  we  know  the  current  flowing  and  the 
resistance  of  the  line.  And  inasmuch  as  the  energy  trans- 
mitted by  the  same  current  varies  directly  with  the  working 
E.  M.  F.,  a  comparison  of  the  loss  in  volts  determined  as 
above,  with  the  initial  E.  M.  F.  applied  to  the  circuit,  shows 
the  percentage  of  energy  lost  in  the  line.  Obviously  its 


THE  LINE.  4*9 

absolute  amount  in  watts  is  equal  to  the  E.  M.  F.  lost,  multi- 
plied by  the  current,  i.  e.,  C  £,  or  from  the  last  equation,  Ca  Rt 
if  we  prefer  to  reckon  in  terms  of  the  resistance. 

Ohm's  law  is  the  basis  of  all  computations  regarding  the 
line,  and  lies  behind  all  the  formulae  used  for  this  purpose. 
The  most  obvious  way  of  applying  it  would  be  to  find  the  re- 
sistance of  the  whole  line  corresponding  to  any  required  cur- 
rent and  loss  of  volts,  and  then  to  look  up  in  a  wire  table  the 
wire  which  when  taken  of  the  required  length  would  give  this 
resistance.  As  a  matter  of  convenience  in  computation  a  large 
number  of  formulae  have  been  devised,  which  include  the  dis- 
tance; loss,  expressed  either  in  volts  or  percentage  lost;  energy 
transmitted,  in  watts,  horse-power  and  the  like;  and  various 
other  factors,  which  may  be  convenient  for  particular  applica- 
tions. Some  of  the  formulae  give  the  total  weight  of  copper 
required,  and  others  the  area  of  the  conductor  expressed  in 
various  ways. 

Of  these  formulae,  all  modified  from  Ohm's  law  to  suit  vari- 
ous conditions,  the  most  generally  convenient  are  those  which 
give  the  area  of  the  required  conductors  in  "circular  mils" 
(/.  c.i  circles  one  one-thousandth  of  an  inch  in  diameter),  a 
barbarous  unit  familiar  in  all  tables  of  wire  sizes,  which  unfor- 
tunately has  obtained  too  firm  a  hold  on  electrical  practice  to 
be  readily  shaken  off. 

Of  these  "circular  mil  "  formulae  one  is  subjoined  which 
has  been  found  by  the  author  to  be  most  convenient  on 
account  of  its  simplicity.  It  is  derived  from  Ohm's  law  as 

E 
follows:  R  —  —FT  •      Now  for  any  length  of  wire  R  = 

Ls 

Total  length  in  ft.  X  resistance  of  i  ft.  wire  i  mil  in  section 
Area  in  circular  mils. 

Taking  the  total  length  of  wire  as  twice  the  distance  of  trans- 
mission in  feet,  since  this  distance  is  particularly  in  mind,  and 
noting  that  the  resistance  of  one  "mil-foot"  of  commercial, 
copper  wire  is  quite  nearly  n  ohms,  we  have 

R  —  2  D  X   TI 

c.  m. 

Substituting  this  value  of  R  in  the  expression  of  Ohm's  law 
just  noted,  and  solving  for  the  value  of  <r.  m.  we  have, 


420 


ELECTRIC    TRANSMISSION  OF  POWER. 


c.  m.  = 


In  this  expression  E  is  the  number  of  volts  lost,  and  n  is 
taken  as  the  "mil-foot"  constant,  since  it  is  large  enough  to 
take  account  of  variations  in  diameter  of  wire,  bad  joints,  flaws, 
and  the  like.  The  theoretical  value  is  nearer  10.8,  but  experi- 
ence has  shown  the  wisdom  of  making  the  above  allowance. 

Having  found  in  any  case  the  area  of  the  required  wire  in 
circular  mils,  its  size  and  weight  per  thousand  feet  can  be 
looked  up  in  any  wire  table. 

Since  it  chances  that  a  wire  of  1,000  c.  m.  weighs  very  nearly 
3  Ibs.  per  thousand  feet,  we  can  obtain  a  very  simple  formula 
giving  directly  the  weight  in  pounds  of  wire  per  thousand  feet. 
Taking  D  in  thousands  of  feet,  and  expressing  this  fact  by 
writing  it  Z>m,  we  have: 

w     _  2  £>m  x  33  X  Ct 
—£-       ~  ' 

or  for  the  total  weight  of  wire 

W  =  4^'m  X  33  X  C 


As  a  matter  of  convenience,  the  following  table  is  inserted, 
giving,  for  the  sizes  of  wire  most  used  in  power  transmission 
work,  the  area  in  circular  mils,  the  diameter,  resistance  per 
thousand  feet,  and  weight  per  thousand  feet,  both  bare  and 
with  insulation  of  the  so-called  "  weather-proof  "  grade,  which 
is  well  adapted  for  ordinary  line  work.  Diameters  are  here 
given  to  the  nearest  mil,  areas  to  the  nearest  10  c.  m.y  and 


Circular  Mils. 

Gauge  No. 
B.  &S. 

toiameter 
in  Mils. 

Resistance  in  ohms 
per  M  feet. 

Wt.  per  M 
feet  Bare. 

Wt.  per  M 
ft.  We'th'r 
proof. 

211,600 

0000 

460 

.04904 

640 

725 

167,800 

000 

4IO 

.06184 

508 

580 

133,100 

00 

365 

.07797 

403 

480 

105,600 

o 

325 

.09827 

320 

375 

83,690 

I 

289 

.12398 

253 

307 

66,370 

2 

258 

.15633 

201 

245 

52,630 

3 

229 

.19714 

159 

195 

41,740 

4 

204 

.24858 

126 

147 

33,ioo 

5 

182 

.31346 

100 

121 

26,250 

6 

162 

•39528 

79 

99 

THE  LTNE.  421 

weights  to  the  nearest  pound.  The  table  is  not  continued 
beyond  the  size  given,  because  anything  larger  than  No. 
.0000  is  exceedingly  troublesome  to  string,  and  when  called 
for  is  better  replaced  with  two  smaller  wires.  On  the  other 
hand,  wire  smaller  than  No.  6  is  seldom  indicated  by  the 
conditions,  and  produces  a  mechanically  weak  line. 

These  formulae  and  table  give  one  sufficient  data  for  comput- 
ing the  general  character  of  a  transmission  line  provided  the 
loss  in  volts  is  determined.  In  actually  working  out  the 
amount  of  copper  needed  in  any  given  case,  certain  details 
require  to  be  taken  into  account  which  will  be  discussed  later. 

A  glance  at  the  formulae  shows  that  the  voltage  employed  is 
the  determining  factor  in  the  cost  of  the  lines.  For  a  fixed 
percentage  of  loss  doubling  the  working  voltage  will  evidently 
divide  the  amount  of  copper  required  by  four,  since  the  cur- 
rent for  a  given  amount  of  energy  will  be  reduced  by  one-half, 
while  the  rolts  lost  will  be  doubled. 

bo  in  general  the  amount  of  copper  required  for  transmit- 
ting a  given  amount  of  energy  a  given  distance  at  a  fixed 
efficiency  will  vary  inversely  as  the  square  of  the  voltage. 

If  the  distance  of  transmission  is  doubled,  the  area  of  the 
conductor  will  evidently  have  to  be  doubled  also;  conse- 
quently, since  the  length  is  doubled,  the  weight  of  copper  will 
be  increased  four  times.  That  is,  for  the  same  energy  trans- 
mitted at  the  same  per  cent,  efficiency  and  the  same  voltage, 
the  weight  of  copper  will  increase  directly  as  the  square  of  the 
distance.  The  advantage  and,  indeed,  necessity  of  employing 
high  voltages  for  transmissions  over  any  considerable  distance 
is  obvious.  In  fact,  it  will  be  seen  that  by  increasing  the 
voltage  in  direct  proportion  to  the  distance,  the  weight  of 
copper  required  for  a  given  percentage  of  loss  will  be  made  a 
constant  quantity  independent  of  the  distance. 

If  one  were  free  to  go  on  increasing  the  voltage  indefinitely 
without  enormously  enhancing  the  electrical  difficulties,  power 
transmission  would  be  a  simple  task,  but  unfortunately  such  is 
not  the  case.  With  very  high  voltages  we  meet  difficulties 
both  in  establishing  and  maintaining  the  insulation  of  the  line, 
and  in  utilizing  the  power  after  it  is  successfully  transmitted. 
The  specific  character  of  these  limitations  will  be  discussed 
later,  but  enough  has  been  said  to  render  it  evident  that  in 
establishing  a  power  transmission  system,  both  the  working 


ELECTRIC    TRANSMISSION   OF  POWER. 

voltage  and  the  volts  lost  in  the  line  must  be  determined  with 
great  judgment. 

In  the  matter  of  economy  in  the  line,  high  voltage  is  desir- 
able— first,  last  and  always.  In  systems  where  the  voltage 
undergoes  no  transformation  its  magnitude  is  somewhat  arbi- 
trarily fixed  by  the  practicable  voltage  which  can  be  employed 
in  the  various  translating  devices,  motors,  lamps  and  the  like. 
For  example,  in  a  system  at  constant  potential  wherein  incan- 
descent lamps  are  an  important  item,  125  volts  or  250  volts 
worked  on  the  three-wire  system  would  be  the  highest  pressure 
advisable  for  the  receiving  system  in  the  present  state  of  the 
art,  or  in  certain  cases  where  cheap  power  can  be  had  these 
voltages  might  be  doubled,  and  220  to  250  volt  lamps  used. 
For  a  direct-current-motor  system  the  corresponding  figure 
would  be  500  to  600  volts  or  1,000  to  1,200  worked  three-wire, 
Similar  limitations  indicated  elsewhere  will  hold  for  other 
classes  of  apparatus. 

When  there  is  a  transformation  of  voltage  in  the  system, 
whether  direct  or  alternating  current,  so  that  the  line  voltage 
is  not  fixed  by  that  of  the  translating  devices,  it  is  advisable  to 
raise  the  voltage  of  transmission  as  high  as  the  existing  state 
of  the  art  permits.  It  must  be  borne  in  mind,  however,  that 
this  general  rule  is  subject  to  modification  by  circumstances. 
It  would  be  bad  economy,  for  instance,  to  use  very  high  pres- 
sures and  costly  insulation  for  a  transmission  of  moderate 
length  and  trifling  magnitude.  Such  practice  would  result  in 
sending  perhaps  100  KW  over  a  line  or  through  a  conduit  which 
could  as  easily  serve  for  ten  times  the  power  without  great 
additional  cost  for  copper.  It  is  well,  however,  not  to  stop  at 
half-way  measures,  but  if  transforming  devices  are  to  be  used 
at  all,  to  go  boldly  to  the  highest  voltage  which  experience 
has  shown  to  be  safe  on  the  line,  or  in  the  generators,  if  only 
reducing  transformers  are  used. 

For  example,  in  most  cases  of  alternating  current  work, 
1,000  volts  is  entirely  obsolete;  if  the  line  voltage  has  to  be 
reduced  at  all  it  is  better  to  get  the  advantage  of  2,000  to 
5,000  volts  on  the  line;  if  raising  and  reducing  transformers 
are  employed  the  latter  figure  might  as  well  be  increased  to 
10,000  or  20,000,  unless  climatic  or  other  special  conditions 
are  unfavorable. 

It  will  be  seen  that  quite  aside  from  engineering   details, 


THE   LINE.  423 

divers  really  commercial  factors  must  enter  into  any  final 
decision  regarding  the  voltage  to  be  used.  And  these  commer- 
cial factors  are  the  final  arbiters  as  to  the  working  voltage,  and 
even  more  completely  as  to  the  proportion  of  energy  which 
it  is  desirable  to  lose  in  the  line.  Power  transmission  systems 
are  installed  to  earn  money,  not  to  establish  engineering  theses. 

It  is  evident,  to  start  with,  that  whatever  the  voltage,  high 
efficiency  of  the  line  and  low  first  cost  are  in  a  measure  mutu- 
ally exclusive.  The  former  means  large  conductors,  the 
latter  small  ones;  the  former  delivers  a  large  percentage  of 
salable  energy,  with  a  high  charge  for  interest  on  line  invest- 
ment; the  latter  a  smaller  amount  of  energy,  with  a  lessened 
interest  account  against  it.  At  first  sight  it  would  seem  easy 
to  establish  'a  relation  between  the  cost  of  energy  lost  on  the 
line  and  the  investment  in  copper  which  would  be  required  to 
save  it,  so  that  one  could  comfortably  figure  out  the  conditions 
of  maximum  economy. 

In  1881  Lord  Kelvin,  then  Sir  William  Thomson,  attacked 
the  problem  and  propounded  a  law,  known  often  by  his  name, 
which  put  the  general  principles  of  the  matter  in  a  very  clear 
light,  but  which  indirectly  has  been  responsible  for  not  a  little 
downright  bad  engineering. 

He  stated,  in  effect,  that  the  most  economical  area  of  con-, 
ductor  will  be  that  for  which  the  annual  interest  charge  equals 
the  annual  cost  of  energy  lost  in  it. 

While  it  is  true  that  for  a  given  current  and  line  Kelvin's 
law  correctly  indicates  the  condition  of  minimum  cost  in  trans- 
mitting said  current,  this  law  has  often  caused  trouble  when 
misapplied  to  concrete  cases  of  power  transmission,  in  that 
it  omits  many  of  the  practical  considerations.  It  involves 
neither  the  absolute  value  of  the  working  voltage  nor  the  dis- 
tance of  transmission,  and  for  long  transmissions  at  moderate 
voltage  often  gives  absurd  values  for  the  energy  lost.  Indeed, 
as  it  deals  directly  only  with  the  most  economical  condition 
for  transmitting  energy,  it  quite  neglects  the  amount  of  energy 
delivered.  In  fact,  one  may  apply  Kelvin's  law  rigidly  to  a 
concrete  and  not  impossible  case,  and  find  that  no  energy  to 
speak  of  will  be  obtained  at  the  end  of  the  line. 

In  other  words,  Kelvin's  law,  while  a  beautifully  correct 
solution  ot  a  particular  problem,  is  in  its  original  form  totally 
inapplicable  to  most  power  transmission  work. 


424  ELECTRIC    TRANSMISSION  OF  POWER. 

Various  investigators,  notably  Forbes,  Kapp,  and  Perrine, 
have  made  careful  and  praiseworthy  attempts  so  to  modify  Kel- 
vin's law  as  to  take  account  of  all  the  facts;  indeed,  nearly  every 
writer  on  power  transmission  has  had  a  shy  at  the  problem. 

Perhaps  the  commonest  attempt  at  improvement  is  to  follow 
the  general  line  of  the  original  law,  but  to  equate  the  interest 
charge  on  copper  to  the  annual  value  of  the  power  lost;  in 
other  words,  to  proportion  the  line  by  increasing  the  copper 
until  the  annual  net  value  of  a  horse  power  saved  in  the  line 
would  be  balanced  by  the  interest  charge  on  the  copper  re- 
quired to  save  it.  This  proposition  sounds  specious  enough 
at  first  hearing.  Practically,  it  produces  a  line  of  far  greater 
first  cost  than  is  usually  justified.  It  is  evident  that  the  pos- 
session of  a  little  extra  power  thus  saved  brings  no  profit 
unless  it  can  be  sold,  and  in  very  few  cases  is  a  plant  worked 
close  enough  to  its  maximum  capacity  during  the  earlier 
years  of  its  existence  to  render  a  trifling  increase  in  out- 
put of  any  commercial  value,  especially  in  the  case  of  trans- 
mission from  water  power.  When  the  plant  is  worked  at  a 
very  high  cost  for  power,  or  soon  reaches  its  full  capacity,  a 
few  horse  power  saved  in  the  line  will  be  valuable;  but  far 
oftener,  particularly  in  water-power  plants,  it  would  be  cheaper 
to  let  the  additional  copper  wait  until  the  necessity  for  it 
actually  arises.  Furthermore,  it  evidently  does  not  pay  to 
so  increase  the  line  investment  that  the  last  increment  of  effi- 
ciency will  bring  no  profit. 

As  an  example,  let  us  suppose  the  case  of  a  1,000  HP  trans- 
mission so  constituted  that  the  line  copper  costs  $10,000 
with  10  per  cent,  loss  of  energy  in  the  line,  and  suppose  in 
addition  that  the  net  value  of  i  HP  at  the  receiving  end  is 
$50  per  annum.  It  is  evident  that  by  decreasing  the  loss  in 
the  line  to  2^  per  cent,  there  would  be  available  75  additional 
HP  worth  $3,750  per  annum.  The  cost  of  this  addition  to 
the  line  would  be  $30,000,  on  which  interest  at  6  per  cent, 
would  be  $1,800.  So  long  as  the  plant  is  not  worked  up  to 
90  per  cent,  of  its  maximum  capacity  of  1,000  HP  there  will 
be  a  steady  charge  of  $r,8oo  plus  depreciation  if  the  additional 
copper  be  installed  at  the  start.  A  few  months'  loss  at  this 
rate  would  more  than  cover  the  labor  of  reinforcing  the  line 
when  needed, -even  supposing  that  installing  the  additional 


THE   L1XE.  425 

copper  at  the  start  would   not  have  involved  extra  labor  in 
construction. 

Various  formulae  for  designing  the  line  so  as  to  secure  the 
minimum  cost  of  transmission  have  been  published,  derived 
more  or  less  directly  from  Kelvin's  law  and  attempting  to 
take  into  account  all  the  various  factors  involved  in  line  effi- 
ciency. They  all  contain  quantities  of  very  uncertain  value, 
and  hence  are  likely  to  give  correspondingly  inexact  results. 
More  than  this,  they  are  founded  on  two  serious  misconceptions. 

First,  they  generally  give  the  minimum  cost  of  transmission, 
which  is  not  at  all  the  same  thing  as  the  maximum  earning 
power  on  the  total  investment.  Second,  however  fully  they 
take  account  of  existing  conditions,  the  data  on  which  they 
are  founded  refer  to  a  particular  epoch,  and  are  very  unre- 
liable guides  in  designing  a  permanent  plant. 

A  few  years  or  even  months  may  and  often  do  so  change  the 
conditions  as  to  lead  to  a  totally  different  result.  In  the  vast 
majority  of  cases  it  is  impossible  to  predict  with  any  accuracy 
the  average  load  on  a  proposed  plant,  the  average  price  to  be 
obtained  for  power,  or  the  average  efficiency  of  the  translating 
devices  which  will  be  used.  So  probable  and  natural  a  thing 
as  competition  from  any  cause,  or  adverse  legislation,  will 
totally  change  the  conditions  of  economy. 

For  these  reasons  neither  Kelvin's  law  nor  any  modification 
thereof  is  a  safe  general  guide  in  determining  the  proper 
allowance  for  loss  of  energy  in  the  line.  Only  in  some  specific 
cases  is  such  a  law  conveniently  applicable.  Each  plant  has 
to  be  considered  on  its  merits,  and  very  various  conditions  are 
likely  to  determine  the  line  loss  in  different  cases.  The  com- 
monest cases  which  arise  are  as  follows,  arranged  in  order  of 
their  frequency  as  occurring  in  American  practice.  Each  case 
requires  a  somewhat  different  treatment  in  the  matter  of  line 
loss,  and  the  whole  classification  is  the  result  not  of  a  priori 
reasoning,  but  of  the  study  of  a  very  large  number  of  concrete 
cases,  embracing  a  wide  range  of  circumstances  and  covering 
a  large  proportion  of  all  the  power  transmission  work  that  has 
been  accomplished  or  proposed  in  this  country. 

CASE  I.  General  distribution  of  power  and  light  from  water 
power.  This  includes  something  like  two-thirds  of  all  the  power 
transmission  enterprises.  The  cases  which  have  been  investi- 


426  ELECTRIC   TRANSMISSION  OF   POWER. 

gated  by  the  author  have  ranged  from  100  to  10,000  HP,  to  be 
transmitted  all  the  way  from  one  to  one  hundred  and  fifty  miles. 
The  market  for  power  and  light  is  usually  uncertain,  the  propor- 
tion of  power  to  light  unknown  within  wide  limits,  and  the  total 
amount  required  only  to  be  determined  by  future  conditions. 
The  average  load  defies  even  approximate  estimation,  and  as 
a  rule  even  when  the  general  character  of  the  market  is  most 
carefully  investigated  little  certainty  is  gained. 

For  one  without  the  gift  of  prophecy  the  attempt  to  figure 
the  line  for  such  a  transmission  by  following  any  canonical 
rules  for  maximum  economy  is  merely  the  wildest  sort  of 
guesswork.  The  safest  process  is  as  follows:  Assume  an 
amount  of  power  to  be  transmitted  which  can  certainly  be 
disposed  of.  Figure  the  line  for  an  assumed  loss  of  energy  at 
full  load  small  enough  to  insure  good  and  easy  regulation, 
which  determines  the  quality  of  the  service,  and  hence,  in 
large  measure,  its  growth.  Arrange  both  power  station  and 
line  with  reference  to  subsequent  increase  if  needed.  The 
exact  line  loss  assumed  is  more  a  result  of  trained  judgment 
than  of  formal  calculation.  It  will  be  in  general  between 
5  and  15  per  cent.,  for  which  losses  generators  can  be  con- 
veniently over  compounded.  If  raising  and  reducing  trans- 
formers are  used  the  losses  of  energy  in  them  should  be 
included  in  the  estimate  for  total  loss  in  the  line.  In  this 
case  the  loss  in  the  line  proper  should  seldom  exceed  10  per 
cent.  A  loss  of  less  than  5  per  cent,  is  seldom  advisable. 

It  should  not  be  forgotten  that  in  an  alternating  circuit  two 
small  conductors  are  generally  better  than  one  large  one,  so 
that  the  labor  of  installation  often  will  not  be  increased  by 
waiting  for  developments  before  adding  to  the  line.  It  fre- 
quently happens,  too,  that  it  is  very  necessary  to  keep  down 
the  first  cost  of  installation,  to  lessen  the  financial  burden 
during  the  early  stages  of  a  plant's  development. 

CASE  II.  Delivery  of  a  known  amount  of  power  from  ample 
water  power.  This  condition  frequently  arises  in  connection 
with  manufacturing  establishments.  A  water  power  is  bought 
or  leased  in  toto,  and  the  problem  consists  of  transmitting 
sufficient  power  for  the  comparatively  fixed  needs  of  the  works. 
The  total  amount  is  generally  not  large,  seldom  more  than 
a  few  hundred  horse  power.  Under  these  circumstances  the 


THE  LINE.  427 

plant  should  be  designed  for  minimum  first  cost,  and  any  loss 
in  the  line  is  permissible  that  does  not  lower  the  efficiency 
enough  to  force  the  use  of  larger  sizes  of  dynamos  and 
water  wheels.  These  sizes  almost  invariably  are  near  enough 
together  to  involve  no  trouble  in  regulation  if  the  line  be  thus 
designed.  The  operating  expense  becomes  practically  a  fixed 
charge,  so  that  the  first  cost  only  need  be  considered. 

Such  plants  are  increasingly  common.  A  brief  trial  calcula- 
tion will  show  at  once  the  conditions  of  economy  and  the  way 
to  meet  them. 

CASE  III.  Delivery  of  a  known  power  from  a  closely  limited 
source.  This  case  resembles  the  last,  except  that  there  is  a 
definite  limit  set  for  the  losses  in  the  system.  Instead,  then, 
of  fixing  a  loss  in  the  line  based  on  regulation  and  first  cost 
alone,  the  first  necessity  is  to  deliver  the  required  power. 
This  may  call  for  a  line  more  expensive  than  would  be  indicated 
by  any  of  the  formulae  for  maximum  economy,  since  it  is  far 
more  important  to  avoid  a  supplementary  steam  plant  entirely 
than  to  escape  a  considerable  increase  in  cost  of  line.  The  data 
to  be  seriously  considered  are  the  cost  of  maintaining  such  a 
supplementary  plant  properly  capitalized,  and  the  price  of  the 
additional  copper  that  will  render  it  unnecessary.  Maximum 
efficiency  is  here  the  governing  factor.  In  cases  where  the 
motive  power  is  rented  or  derived  from  steam,  formulae  like 
Kelvin's  may  sometimes  be  convenient.  Losses  in  the  line 
will  often  be  as  low  as  5  per  cent.,  sometimes  only  2  or  3. 

CASE  IV.  Distribution  of  power  in  known  amount  and  units, 
with  or  without  long-distance  transmission,  with  motive  power 
which,  like  steam  or  rented  water  power,  costs  a  certain 
amount  per  horse-power.  Here  the  desideratum  is  minimum 
cost  per  HP,  and  design  for  this  purpose  may  be  carried  out 
with  fair  accuracy.  Small  line  loss  is  generally  desirable 
unless  the  system  is  complicated  by  a  long  transmission. 
Such  problems  usually  or  often  appear  as  distributions  only. 
Where  electric  motors  are  in  competition  with  distribution  by 
shafting,  rope  transmission,  and  the  like,  2  to  5  per  cent,  line 
loss  may  advantageously  be  used  in  a  trial  computation. 

The  problem  of  power  transmission  may  arise  in  still  other 
forms  than  those  just  mentioned.  Those  are,  however,  the 
commonest  types,  and  are  instanced  to  show  how  completely 


428  ELECTRIC    TRANSMISSION   OF  POWER. 

the  point  of  view  has  to  change  when  designing  plants  under 
various  circumstances.  The  controlling  element  may  be 
minimum  first  cost,  maximum  efficiency,  minimum  cost  of 
transmission  or  combinations  of  any  one  of  these,  with  locally 
fixed  requirements  as  to  jne  or  more  of  the  others,  or  as  to 
special  conditions  quite  apart  from  any  of  them. 

In  very  many  cases  it  is  absolutely  necessary  to  keep  down 
the  initial  cost,  even  at  a  considerable  sacrifice  in  other  respects. 
Or  economy  in  a  certain  direction  must  be  sought,  even  at  a 
considerable  expense  in  some  other  direction.  For  these 
reasons  no  rigid  system  can  be  followed,  and  there  is  constant 
necessity  for  individual  skill  and  judgment.  It  is  no  uncom- 
mon thing  to  find  two  plants  for  transmitting  equal  powers 
over  the  same  distance  under  very  similar  conditions,  which 
must,  however,  be  installed  on  totally  different  plans  in  order 
to  best  meet  the  requirements. 

As  regards  the  general  character  of  transmission  lines  the 
most  usual  arrangement  is  to  employ  bare  copper  wire  sup- 
ported on  wooden  or  iron  poles  by  suitable  insulators.  Now 
and  then  underground  construction  becomes  necessary  owing 
to  special  conditions.  Not  infrequently  an  aerial  transmission 
line  must  be  coupled  with  underground  distribution  owing  to 
municipal  regulations.  Occasionally  insulated  line  wire  is 
used.  It  is  frequently  employed  in  cases  where  the  transmis- 
sion lines  are  continued  for  purposes  of  distribution  through 
the  streets  of  a  town,  in  fact,  is  usually  required.  As  such 
lines  are  generally  of  moderate  voltage,  very  seldom  exceed- 
ing 3,000  volts,  good  standard  insulation  may  often  be  effective1 
in  lessening  the  danger  to  life  in  case  of  accidental  contacts, 
and  in  reducing  the  trouble  from  crossing  of  the  lines  with 
other  lines,  branches  of  trees,  and  the  like.  In  case  of  really 
high  voltages,  5,000  to  10,000  and  upward,  no  practicable 
insulation  can  be  trusted  for  the  former  purpose,  and  may  in 
fact  create  a  false  sense  of  security,  while  it  is  far  better 
practice  to  endeavor  to  avert  the  danger  of  short  circuits  than 
to  take  extraordinary  precautions  to  mitigate  their  momentary 
severity.  Hence  bare  copper  is  to  be  preferred  both  on  the 
score  of  safety  and  of  economy.  Now  and  then  at  some  par- 
ticular point  a  high  grade  of  insulation  may  minimize  local 
difficulties. 


THE   LINE.  429 

Much  can  be  said  in  favor  of  placing  a  transmission  line 
underground,  but  there  are  also  very  strong  reasons  against  it. 
Such  a  line  is  eminently  safe,  and  free  from  danger  of  acci- 
dental injury.  At  the  same  time  it  is  very  difficult  to  insulate 
properly,  and  if  trouble  does  arise  it  is  exceedingly  hard  to 
locate  and  difficult  to  remedy.  In  addition,  there  are  serious 
electrical  difficulties  .to  be  encountered,  which  often  can  be 
reduced  only  by  very  costly  construction.  The  chief  objec- 
tion aside  from  these  is  the  expense,  which  in  very  many  cases 
would  be  simply  prohibitive. 

In  cities  there  is  an  increasing  tendency  on  the  part  of  the 
authorities  to  demand  underground  construction.  Overhead 
wires  are  objectionable  on  account  of  their  appearance,  danger 
to  persons  and  property,  and  their  great  inconvenience  in 
cases  of  fire,  and  these  objections  apply  with  almost  equal 
total  force  to  all  such  wires,  whether  used  for  electric  light  or 
power,  or  for  telegraphic  and  telephonic  purposes,  the  latter 
more  than  making  up  by  their  number  for  any  intrinsic  advan- 
tage in  the  matter  of  safety.  The  future  city  will  have  its 
electric  service  completely  underground,  at  least  in  the  more 
densely  inhabited  portions.  It  must  be  said,  however,  that  it 
is  far  more  important  for  a  city  to  have  electric  light  and 
power  than  to  insist  on  having  it  in  a  particular  way,  and 
unless  the  service  is  very  dense,  so  as  to  abundantly  justify 
the  very  great  added  cost  of  underground  work,  private 
capital  will  hesitate  to  embark  in  an  enterprise  so  financially 
overloaded. 

Fortunately,  for  city  distribution  moderate  voltages  must  be 
employed  on  account  of  the  intrinsic  limits  of  direct  current 
circuits  employed  for  general  distribution,  and  the  undesira- 
bility  of  distributing  transformers  of  moderate  size  on  very 
high  pressure  alternating  circuits.  More  than  2,000  to  2,500 
volts,  save  on  arc  circuits,  can  seldom  be  used  advantageously 
in  general  distribution,  and  such  voltages  can  be  and  are  suc- 
cessfully insulated  without  prohibitive  expense.  They  work 
well  in  practice,  and  have  stood  the  test  of  considerable  expe- 
rience. Moreover,  with  proper  care  the  cables  employed  as 
conductors,  when  thoroughly  protected  and  inspected,  probably 
have  a  slightly  less  rate  of  depreciation  than  overhead  insulated 
lines,  and  are  much  less  liable  to  interruption.  As  the  district 


43°  ELECTRIC   TRANSMISSION  OF  POWER. 

within  which  underground  service  is  necessary  is  usually  of  no 
great  extent,  the  electrical  difficulties  that  are  to  be  dreaded 
in  attempting  long  underground  transmissions  are  not  here  of 
so  serious  magnitude. 

For  this  limited  service,  then,  in  districts  where  both  popu- 
lation and  service  are  dense,  there  is  no  serious  objection  to 
underground  lines,  and  many  who  have  used  them  are  decided 
in  commending  them  as  on  the  whole  more  convenient  and 
reliable  than  aerial  lines.  Besides,  a  large  proportion  of  under- 
ground work  is  done  at  low  voltages,  less  than  250  volts,  with 
which  the  difficulties  of  insulation  except  at  joints  are  really 
trivial.  Such  work  does  not  belong  so  much  to  power  trans- 
mission proper  as  to  distribution  from  centers  after  the  trans- 
mission is  accomplished. 

With  high  voltages  and  long  distances  the  case  is  very  dif- 
ferent. Not  only  are  the  difficulties  of  insulation  great,  but 
electrical  troubles  are  introduced  of  so  severe  a  character  as  to 
make  success  very  problematical,  even  in  cases  where  the  cost 
alone  is  not  prohibitive.  The  feat  of  cable  insulation  for  pres- 
sures as  great  as  10,000  volts  has  been  accomplished,  and  this 
limit  could  probably  be  exceeded,  but  the  cost  of  such  work  is 
necessarily  extremely  high,  and  the  location  and  repair  of 
faults  is  troublesome.  An  overhead  line  is  so  much  easier  to 
insulate  and  to  maintain  that  nearly  all  power  transmission 
will  probably  continue  to  be  carried  on  by  this  method  for  some 
time  to  come,  until,  indeed,  there  are  revolutionary  changes 
in  underground  work  of  which  we  now  have  no  suggestion. 
The  possibility  of  a  long  interruption  of  service  while  a  fault 
is  found  and  repaired  is  too  unpleasant  a  contingency  to  be 
incurred.  Duplicate  lines  are  a  natural  recourse  in  such  case, 
effective,  but  very  costly.  Aerial  lines  are  much  cheaper  to 
duplicate,  and  the  labor  of  finding  and  repairing  faults  is  com- 
paratively light.  Finally  when  it  comes  to  the  question  of 
really  high  voltages  like  those  now  coming  into  frequent  use, 
say  30,000  to  60,000  volts,  it  must  be  admitted  that  in  the 
present  state  of  the  art  of  insulation  underground  cables,  if 
possible  at  all,  are  absolutely  prohibitive  in  cost. 

For  these  reasons  underground  transmission  lines  should  be 
avoided,  certainly  until  we  have  had  a  long  experience  with 
high  voltages  overhead. 


THE  LINE. 


431 


Throughout  the  foregoing  it  has  been  assumed  that  the  con- 
ducting line  is  composed  of  the  best  quality  of  commercial 
copper  wire.  Inasmuch  as  other  materials  are  occasionally 
proposed,  it  is  worth  while  saying  something  about  the  relative 
properties  of  certain  metals  and  alloys  as  conductors.  Aside 
from  silver,  pure  copper  is  intrinsically  the  best  conductor 
among  the  metals.  In  fact,  it  is  hard  to  say  that  it  is  not  the 
equal  of  silver.  Commercial  copper  wire  is  of  somewhat  vari- 
able conductivity,  since  this  property  is  profoundly  affected  by 
very  small  proportions  of  certain  other  substances.  It  used  to 
be  a  most  difficult  matter  to  procure  commercial  wire  of  good 
quality,  and  in  the  early  days  of  telegraphy  much  annoyance 
was  experienced  on  this  score.  At  present  the  best  grades  of 
standard  copper  wire  have  a  conductivity  of  fully  98  per  cent, 
that  of  the  chemically  pure  metal,  and  even  this  figure  is  not 
infrequently  exceeded.  On  account  of  the  comparatively  low 
tensile  strength  of  copper,  ordinarily  about  35,000  Ibs.  per 
square  inch,  very  vigorous  efforts  have  been  made  to  exploit 


MATERIAL. 

CONDUCTIVITY. 

STRENGTH,  LBS. 

Commercial  copper  wire  
Good  hard-  drawn  copper.  .  .  . 

98-99 
96-97 

Q7 

35,ooo 
60,000-65,000 
63  200 

(2)  Silicon  bronze     .  .            .  . 

80 

76  ooo 

Aluminium               

59-6O 

32,900 

45 

110,000 

Phosphor  bronze 

26 

IOI  OOO 

Iron  annealed  wire       .        .... 

14 

55  ooo 

High  carbon  steel  wire  

IO-I2 

120,000-1  30,000 

various  alloys  of  copper  on  the  theory  that  their  greater 
strength  would  more  than  overbalance  the  lessened  conduc- 
tivity and  increased  cost,  by  enabling  less  frequent  supports  to 
be  employed.  Aluminium  bronze,  silicon  bronze  and  phos- 
phor bronze  have  been  tried,  together  with  some  other  alloys 
of  a  similar  character  exploited  under  various  trade  names. 
The  whole  matter  of  high  conductivity  bronzes  has  been  so 
saturated  with  humbug  that  it  is  very  hard  indeed  to  get  at  the 
facts  in  the  case.  Copper  which  is  hard-drawn  probably  has 
greater  tensile  strength  than  any  so-called  bronze  of  similar 


43 2  ELECTRIC   TRANSMISSION   OF  POWER. 

conductivity,  from  60,000  to  70,000  Ibs.  per  square  inch,  with 
a  resistance  less  than  5  per  cent,  in  excess  of  that  of  ordinary 
copper.  The  foregoing  table  gives  the  conductivities  and  ten- 
sile strengths  of  some  of  the  various  materials  used  or  proposed 
for  line  wire,  pure  copper  being  taken  as  the  standard  at  100 
per  cent,  conductivity. 

It  is  sufficiently  evident  from  this  table  that  where  the  best 
combination  of  strength  and  conductivity  is  wanted,  hard-drawn 
copper  is  unexcelled.  For  all  ordinary  line  work  good  annealed 
copper  wire  is  amply  strong,  and  is,  besides,  easier  to  manip- 
ulate than  wire  of  greater  hardness.  Occasionally,  where  it 
it  desirable  to  use  extra  long  spans,  or  excessive  wind  pressure 
is  to  be  encountered,  the  hard-drawn  wire  is  preferable.  Now 
and  then,  in  crossing  rivers  or  ravines,  spans  of  great  length 
are  desirable, — several  hundred  yards, — and  in  these  cases  one 
may  advantageously  employ  silicon  or  other  bronze  of  great 
tensile  strength,  or  as  an  alternative  a  bearer  wire,  preferably 
a  steel  wire  cable,  carrying  the  copper  conducting  wire  or 
itself  serving  as  the  conductor.  Where  mechanical  strains  are 
frequent  and  severe,  bronzes  are  somewhat  more  reliable  than 
hard-drawn  copper  of  equal  tensile  strength,  since  they  are 
homogeneous,  while  the  hard-drawn  copper  owes  much  of  its 
increase  in  tenacity  to  a  hard  exterior  shell,  the  core  of  it 
being  substantially  like  ordinary  copper. 

Compound  wires  have  now  and  then  been  used,  consisting  o* 
a  steel  core  with  a  copper  covering,  but  these  are  costly  and 
no  better  than  hard-drawn  copper  for  line  use.  Iron  alone 
replaces  copper  to  any  extent.  It  is  cheaper  for  equal  conduc- 
tivity, but  in  wire  is  far  less  durable,  and  in  rods  cannot  be 
strung  overhead  conveniently,  while,  even  were  this  possible, 
the  difficulty  of  making  and  maintaining  joints  is  most  serious. 
Very  recently  aluminium  has  been  successfully  used  as  a  line 
conductor.  At  present  prices  (1901)  it  is  materially  cheaper 
than  copper  for  equal  conductivity,  but  its  bulk  and  the  diffi- 
culty of  making  joints  are  objectionable.  Aluminium  has 
about  six-tenths  the  conductivity  of  copper,  the  resistance  of 
one  mil-foot  of  pure  aluminium  wire  being  17.6  ohms  at  25°  C. 
Owing  to  its  very  low  specific  gravity  its  conductivity  is  very 
high  when  compared  on  the  basis  of  weight.  It  has  just  one- 
half  the  weight  of  copper  for  the  same  conductivity,  so  that  as 


THE   LINE.  433 

a  conductor  aluminium  wire  at  30  cents  per  pound  is  as  cheap 
as  copper  wire  at  15  cents  per  pound.  The  tensile  strength 
of  the  aluminium  is  slightly  less  than  that  of  copper,  being  a 
little  less  than  33,000  Ibs.  per  square  inch,  while  soft-drawn 
copper  is  about  2,000  Ibs.  higher.  Like  copper,  the  aluminium 
wire  takes  permanent  set  very  easily,  having  a  very  low  elastic 
limit,  and  at  .about  half  its  ultimate  strength  is  apt  to  stretch 
seriously.  Comparing  wires  of  equal  conductivity  the  aluminium 
has  absolutely  greater  strength,  since  its  cross-section  is  about 
1.64  times  that  of  the  corresponding  copper  wire.  If,  however, 
the  copper  be  hard  drawn,  the  aluminium  wire  of  the  same 
conductivity  is  only  about  ^  as  strong. 

Being  somewhat  larger,  the  aluminium  wire  has  a  trifle 
greater  inductance  and  capacity  than  the  copper  and  is  more 
exposed  to  the  effect  of  storms.  It  has  about  1.4  times  the 
linear  coefficient  of  expansion  of  copper,  so  that  there  is  more 
tendency  to  sag  in  hot  weather  and  to  draw  dangerously  taut 
in  cold  weather.  This  property  has  caused  some  practical 
trouble  in  aluminium  lines,  and  has  to  be  met  by  great  atten- 
tion to  temperature  and  uniform  tension  in  stringing  the  wire. 

Joints  in  aluminium  wire  are,  as  already  indicated,  a  very 
serious  problem.  In  contact  with  other  metals  aluminium  is 
attacked  electrolytically  by  almost  everything,  even  zinc.  A 
•successful  soldered  joint  for  aluminium  has  not  yet  been  pro- 
duced, and  in  line  construction  recourse  has  to  be  taken  to 
mechanical  joints.  One  of  the  most  successful  of  these  is  that 
used  in  several  California  lines.  It  consists  of  an  oval  aluminium 
sleeve,  large  enough  to  slip  in  the  two  wire  ends  side  by  side,  and 
for  No.  i  wires  about  9"  long.  In  making  the  joint  the  ends  of 
the  wires  were  filed  rough,  the  wires  were  slipped  side  by  side 
through  the  sleeve,  and  then  by  a  special  tool  sleeve  and  wires 
were  twisted  through  two  or  three  complete  turns.  The  result 
was  a  joint  practically  as  strong  as  the  original  wire,  and 
electrically  good.  There  is  considerable  danger  of  electrolytic 
corrosion  in  any  such  mechanical  joint,  and  lines  exposed  to 
^alt  fogs  would  probably  suffer  rather  severely  in  this  way,  but 
with  care  in  making,  and  regular  inspection,  these  joints  serve 
the  purpose  well.  Very  recently  a  process  of  cold  welding  a 
•sleeve  joint  under  great  pressure  has  given  excellent  results. 

Altogether   it    seems   likely    that   aluminium  may  serve  as 


434  ELECTRIC   TRANSMISSION   OF  POWER. 

a  most  useful  substitute  for  copper  for  transmission  lines, 
and  it  will  certainly  be  used  extensively  if  copper  holds 
to  a  price  of  17  to  19  cents  per  pound  for  bare  wire.  Not 
only  is  the  aluminium  cheaper  in  first  cost,  but  its  lesser  weight 
means  a  great  decrease  in  cost  of  freights  as  well.  It  cer- 
tainly makes  an  excellent  line  when  carefully  put  up,  and  there 
is  no  good  reason  why  it  should  not  be  freely  used  whenever 
the  price  of  copper  throws  the  balance  of  economy  in  favor  of 
aluminium.  There  have  been  attempts  to  improve  the  strength 
of  aluminium  wire  by  alloying  it,  but  as  in  the  case  of  bronzes 
the  gain  in  strength  is  at  the  expense  of  conductivity.  Such 
alloy  wire  should  be  very  cautiously  investigated  before  use. 

Before  taking  up  the  practical  task  of  line  calculation  it  is 
necessary  to  consider  somewhat  at  length  the  electrical  diffi- 
culties that  must  be  encountered,  and  which  impose  limitations 
on  our  practically  achieving  many  things  that  in  themselves 
are  desirable  and  useful.  We  have  seen  already  that  the  secret 
of  long  distance  transmission  lies  in  the  successful  employ- 
ment of  very  high  voltages,  and  whatever  the  character  of  the 
current  employed,  the  difficulties  of  insulation  constantly  con- 
front us.  These  are  of  various  sorts,  for  the  most  part,  how- 
ever, those  that  have  to  do  with  supporting  the  conducting 
line  so  that  there  may  not  be  a  serious  loss  of  current  via  the 
earth.  Next  in  practical  importance  come  those  involved  in 
insulating  the  conductor  as  a  whole  against,  first,  direct  earth 
connections  or  short  circuits  in  underground  service,  and 
second,  grounds  or  short  circuits,  if  the  line  is  an  aerial  one. 

In  a  very  large  number  of  cases  no  attempt  is  made  to 
insulate  the  wire  itself  by  a  continuous  covering,  and  reliance 
is  placed  entirely  on  well-insulated  supports.  In  most  high 
voltage  lines  this  is  the  method  employed,  partly  for  economy 
but  chiefly  because  there  is  well  grounded  distrust  in  the  du- 
rability of  any  practicable  continuous -covering  under  varying 
climatic  conditions  and  the  constant  strain  imposed  by  high 
voltage  currents. 

So  far  as  supports  go,  it  is  evident  that  while  the  individual 
resistance  of  any  particular  one  may  be  very  great,  the  total 
resistance  of  all  those  throughout  the  extent  of  a  long  line  to 
which  they  are  connected  in  parallel  to  the  earth,  may  be  low 
enough  to  entail  a  very  considerable  total  loss  of  energy.  The 


THE  LINE.  435 

possibility  of  such  loss  increases  directly  with  the  number  of 
supports  throughout  the  line.  The  most  obvious  way  of 
reducing  such  losses  would  be  to  considerably  increase  the 
distance  between  supports.  This  process  evidently  cannot  go 
on  indefinitely,  from  mechanical  considerations,  and  hence  the 
greatest  advance  can  be  made  in  reducing  the  chance  of  loss  in 
individual  supports. 

Most  of  the  present  practice  consists  merely  of  an  extension 
of  the  methods  that  were  devised  for  telegraphic  work.  These 
were  quite  sufficient  for  the  purpose  intended,  but  are  inade- 
quate when  applied  to  modern  high  voltage  work. 

The  ordinary  line  consists,  then,  of  poles,  bearing  on  pins  of 
wood  or  metal  secured  to  cross  arms,  bell-shaped  glass  or  por- 
celain insulators.  To  grooves  on  or  near  the  top  of  these  the 
line  wire  is  secured  by  binding  wire.  Loss  of  current  to  earth 
in  a  line  so  constituted  takes  place  in  two  ways.  First,  the 
current  may  pass  over  the  outer  surface  of  the  insulator,  up 
over  the  interior  surface,  thence  to  the  supporting  pin  and  so 
to  earth.  Second,  it  may  actually  puncture  the  substance  of 
the  insulator  and  pass  directly  to  the  supporting  structure. 

The  first  source  of  trouble  is  the  commoner,  and  depends  on 
the  nature  and  extent  of  the  insulating  surface,  and  even  more 
on  climatic  conditions.  The  second  depends  on  the  thickness 
and  quality  of  the  insulating  wall  which  separates  the  wire 
from  the  pin.  To  avoid  leakage  an  insulator  should  be  so 
designed  that,  first,  the  extent  of  surface  shall  be  as  long  and 
narrow  as  possible;  second,  that  this  surface  shall  be  both 
initially  and  continuously  highly  insulating.  The  first  condi- 
tion is  met  by  making  the  insulator  of  comparatively  small 
diameter,  and  adding  to  the  length  of  the  path  over  which  leak- 
age must  take  place  by  placing  within  the  outer  bell  of  the 
insulator  one  or  more  similar  bells  (usually  called  petticoats). 
These  not  only  help  in  the  way  mentioned,  but  they  are  likely 
to  stay  tolerably  dry  even  when  the  exterior  surface  is  wet,  and 
thus  help  to  maintain  the  insulation. 

A  good  glass  or  porcelain  insulator  made  on  these  general 
lines  gives  excellent  results  with  ordinarily  moderate  voltages, 
say  up  to  5,000  volts.  When  the  insulators  are  new  and  clean 
they  will  quite  prevent  perceptible  leakage,  and  for  the  volt- 
ages mentioned  are  satisfactory  under  all  ordinary  conditions. 


436  ELECTRIC    TRANSMISSION  OF  POWER. 

When  higher  voltages  are  employed  the  results  may  be  at  first 
good,  but  they  are  unlikely  to  stay  so  unless  the  climatic  con- 
ditions are  exceptionally  favorable.  Most  glass  permits  a 
certain  amount  of  surface  leakage,  even  when  new,  although 
generally  not  enough  to  be  of  practical  importance,  but  even 
the  best  glass  weathers  when  exposed  to  the  elements,  so  that 
in  time  the  surface  becomes  slightly  roughened  and  retains  a 
film  of  dirt  and  moisture  that  is  a  very  tolerable  conductor. 
Even  while  perfectly  free  from  this  deterioration  at  first,  it  is 
generally  hygroscopic,  because  it  is  in  a  trifling  degree  soluble 
even  in  rain  water,  and  tends  to  retain  a  slight  amount  of 
moisture.  Thus  in  damp  climates  glass  is  likely  to  give 
trouble  when  used  on  a  high  voltage  line.  As  regards  tem- 
porary fall  in  insulating  properties,  a  searching  fog  or  drizzling 
rain  is  worse  in  its  effects  on  insulators  than  a  sharp  shower  or 
even  a  heavy  rain,  which  tends  to  wash  the  outer  surface  free 
of  dirt,  and  affects  the  comparatively  clean  interior  but  little. 

Much  cheap  porcelain  is  also  hygroscopic  owing  to  the  poor 
quality  of  the  glaze,  and  it  has  the  considerable  added  dis- 
advantage of  depending  on  this  glaze  for  much  of  its  insulat- 
ing value.*  Glass  is  homogeneous  throughout  its  thickness, 
while  porcelain  inside  the  glaze  is  often  porous  and  practically 
without  insulating  value.  Nevertheless  porcelain  which  is 
thoroughly  vitrified,  the  ordinary  glaze  being  replaced  by  an 
actual  fusing  of  the  surface  of  the  material  itself,  is  decidedly 
preferable  to  glass,  being  tough  and  strong,  quite  non-hygro- 
scopic, and  of  very  high  insulating  properties.  The  surface 
does  not  weather,  and  the  insulation  is  well  kept  up  under  all 
sorts  of  conditions.  If  the  vitrification  extends,  as  it  should, 
considerably  below  the  surface,  the  insulator  will  resist  not 
only  leakage,  but  puncture,  better  than  any  glass.  The  proc- 
ess of  making  this  quality  of  porcelain  is  somewhat  costly, 
since  the  baking  has  to  be  at  an  enormous  temperature  and 
long  continued,  but  the  result  is  the  most  efficient  insulating 
substance  in  use.  Glass,  however,  is  better  than  ordinary 
grades  of  porcelain. 

Surface  leakage  is  more  to  be  feared  than  puncture  at  all 

*  Much  American  porcelain  will  absorb  i  to  2  per  cent,  of  its  weight  of  water,  a  sign  of  poor 
insulating  properties.  The  best  porcelain  should  absorb  no  water  and  should  show  a  brilliant 
vitreous  fracture  which  will  take  no  flowing  stain  from  ink. 


THE   LINE.  437 

voltages,  since  the  absolute  insulation  strength  of  the  material 
can  be  made  high  enough,  by  careful  attention  to  quality  and 
sufficient  thickness,  to  withstand  any  practical  voltage  contin- 
uously, barring  mechanical  injury.  But  leakage  is  a  function 
of  moisture,  drifting  dust,  and  things  meteorological  generally, 
besides  which,  it  may  take  place  in  serious  amount  at  voltages 
which  otherwise  would  be  very  easy  to  work  with. 

As  the  result  of  all  recent  experiments,  it  may  be  confidently 
stated  that  danger  from  actual  breaking  down  of  the  insulation 
by  puncture  need  not  be  at  all  feared  up  to  certainly  25,000 
volts  with  any  good  modern  high  voltage  insulator.  With  the 
best  insulators  at  present  attainable  the  voltage  just  men- 
tioned can  easily  be  doubled  without  serious  danger  of  break- 
ing through  the  material. 

At  such  high  electrical  pressures,  however,  the  difficulty  of 
surface  leakage  becomes  formidable,  especially  in  bad  weather. 
Glass  insulators  become  troublesome,  and  only  the  best  glass  or 
porcelain  is  to  be  trusted — even  this  with  some  reservations. 
To  reinforce  the  insulation  of  the  surface  it  has  been  sometimes 
deemed  advisable  to  take  recourse  to  the  insulating  properties 
of  heavy  mineral  oil.  Imagine  the  lower  edge  of  the  outer  bell 
of  any  insulator  folded  inward  so  as  to  form  a  deep  groove 
opening  upward  all  around  the  insulator.  Fill  this  hollow 
with  oil,  and  it  is  evident  that  if  leakage  takes  place  at  all  it 
must  be  across  the  surface  of  this  oil. 

Such  an  oil  insulator  is  quite  free  from  surface  leakage  so 
long  as  the  oil  surface  is  kept  clean  and  in  good  condition. 
This  is,  however,  very  difficult  to  do  and  there  is  great  danger 
that  the  oil  surface,  from  the  combined  action  of  dirt  and  mois- 
ture, will  degenerate  into  a  sp  jcies  of  conducting  slime.  Where 
dust  is  very  prevalent  the  oil  is  specially  likely  to  give 
trouble  ;  it  is  far  better  able  to  cope  with  moisture  alone  than 
with  moisture///^ dust.  For  this  reason  the  use  of  oil  insula- 
tors has  been  now  practically  abandoned.  From  what  has  been 
said  it  is  clear  that  line  insulation  is  largely  a  matter  of  climate. 
Where  it  is  uniformly  warm  and  dry  almost  any  kind  of  in- 
sulator will  suffice,  provided  the  material  has  tolerable  insula- 
tion strength  and  the  insulator  is  of  passably  good  design. 
With  a  climate  foggy  in  summer  and  sleety  in  winter,  even 


438  ELECTRIC  TRANSMISSION  OF  POWER. 

the  best  porcelain  insulators  will  be  severely  tried  at  high 
voltages. 

The  record  of  what  has  actually  been  done  in  the  transmis- 
sion of  electrical  energy  at  high  voltages  is  comparatively 
short,  but  has  now  given  invaluable  data  for  the  important 
work  over  long  distances  that  must  be  done  in  the  next  few 
years.  The  experience  with  arc  light  circuits  at  5,000  volts  or 
above,  working  over  lines  from  20  to  40  miles  long,  has  been 
accumulating  for  ten  years.  Seventy-five  to  150  light  dynamos, 
are  now  common,  the  latter  giving  in  the  vicinity  of  10,000 
volts,  and  this  from  open-coil  armatures  of  which  the  maximum 
voltage  is  roughly  equal  to  that  of  an  alternator  of  the  same 
nominal  voltage.  The  lines  are  usually — almost  always— of 
wire  having  no  very  high  insulating  properties,  supported  on. 
common  glass  insulators,  often  without  more  than  a  single 
interior  petticoat.  One  hundred  and  fifty  light  genera- 
tors are  in  occasional  use,  and  now  and  then  two  machines 
of  75  lights  capacity  or  more  are  operated  in  series  for  con- 
siderable periods.  This  practice  is  commoner  than  is  generally 
supposed. 

Alternating  circuits  of  5,000  volts  or  more  are  now  very  com- 
mon here  and  abroad.  Scores  of  them  have  been  in  operation, 
long  enough  to  be  thoroughly  tested.  An  1 8-mile  transmission 
at  a  little  over  5,000  volts  to  Guadalajara,  Mexico;  an  n-mile 
6,000  volt  line  to  Hartford,  Conn.,  and  the  Tivoli-Rome  plant, 
18  miles  at  5,000  volts,  have  now  been  in  steady  and  successful 
operation  for  nearly  a  decade.  A  number  of  others,  probably  a 
score  in  all,  of  less  prominence,  owing  to  shorter  length  of 
lines  and  smaller  capacity,  have  been  for  several  years  in 
regular  operation  at  5,®oo  to  6,000  volts.  Most  of  the  lines 
are  of  bare  copper  wire  supported  on  plain  insulators  of  glass 
or  porcelain.  They  have  been  uniformly  free  from  all  serious 
trouble. 

In  the  vicinity  of  10,000  volts  the  installations  are  now  numer- 
ous and  entirely  conclusive.  Probably  a  hundred  commercial 
plants  are  to-day  actually  operated  at  such  pressures.  One  is 
the  well-known  Ferranti  station,  working  a  io,ooo-volt  main 
from  Deptford  to  London,  about  n  miles,  and  using  a  concen- 
tric underground  cable.  The  plant  has  experienced  various. 


THE  LINE.  439 

vicissitudes  and  has  not  been  regularly  operated,  but  the 
troubles  are  not  generally  chargeable  to  the  mains,  which  have, 
however,  been  a  little  uncertain  in  their  performance.  Another 
early  case  is  the  lighting  plant  in  San  Antonio  Canon,  Cali- 
fornia. This  consists  of  a  pair  of  alternating  generators  of  150 
KW  at  1,000  volts.  Raising  transformers  establish  a  line  pres- 
sure of  10,000  volts,  at  which  current  is  transmitted  to  Pomona, 
16  miles  distant,  and  to  San  Bernardino,  28  miles.  At  each  of 
these  places  is  a  sub-station  with  reducing  transformers,  and 
regulating  apparatus.  The  current  is  used  exclusively  for 
lighting,  and  the  plant  has  been  in  thoroughly  successful  opera- 
tion for  about  eight  years.  The  line  is  of  bare  copper  sup- 
ported on  good-sized,  double-petticoat  glass  insulators  without 
oil.  There  has  been  practically  no  trouble  from  leakage,  even 
during  the  winter,  when  rains  are  of  almost  daily  occurrence, 
no  insulators  have  been  punctured,  and  the  only  trouble  on 
the  line  has  been  of  a  very  trifling  character,  and  due  to  acci- 
dental causes,  such  as  a  tree  branch,  an  occasional  insulator 
broken  by  a  charge  of  shot  and  the  like.  The  line  has  been 
worked  long  enough  to  develop  any  probable  trouble,  and  is 
itself  a  sufficient  demonstration  of  the  entire  practicability  of 
io,ooo-volt  transmission  of  energy.  A  third  case  is  the  Fol- 
som  Sacramento  plant  in  California,  which  has  now  operated 
at  11,000  volts  for  the  last  six  years  without  material  trouble. 
Other  plants  at  Niagara  Falls,  Fresno,  Cal.,  Salt  Lake  City, 
Pachuca,  Mexico,  and  a  few  other  points  have  run  suc- 
cessfully at  10,000  to  11,000  volts  for  five  years  or  more.* 

Of  systems  operated  at  much  more  than  10,000  volts  there 
are  at  present  a  good  many,  and  experiments  have  been  fre- 
quent and  generally  successful.  The  most  noted  of  these  is  the 
Lauffen-Frankfort  line,  108  miles  long  (three-phase),  worked 
somewhat  irregularly  during  the  latter  part  of  the  summer  of 
1891.  Operations  were  generally  at  13,800  volts,  though  on  a 
few  occasions  this  was  temporarily  doubled.  There  was  no 
noticeable  leakage,  but  an  insulator  was  now  and  then  punc- 
tured even  at  the  lower  pressure,  producing  a  tiny,  irregular 
hole  clear  through  from  the  neck  of  the  insulator  to  the  sup- 
porting pin.  The  insulators  which  supported  the  bare  copper 
line  were  of  porcelain  with  oil  grooves.  The  line  worked 


44°  ELECTRIC    TRANSMISSION  OF  POWER. 

well  in  all  sorts  of  weather,  but  the  total  period  of  oper- 
ation was  too  short  to  give  this  brilliant  experiment  much 
value  as  a  precedent.  A  three-phase  transmission  at  14,000 
volts,  14  miles  to  the  works  of  the  Oerlikon  Company  near 
Zurich,  has  been  in  successful  operation  for  the  past  nine 
years. 

Most  of  the  large  electrical  manufacturing  companies  have 
during  the  last  few  years  carried  on  experiments  on  high-volt- 
age transmission,  mostly  with  short  lines  near  their  factories. 
The  range  of  voltages  has  varied  from  10,000  to  25,000  or 
even  50,000,  and  the  general  experience  has  been  that  at  the 
lower  pressure  mentioned  successful  working  can  be  attained 
under  almost  any  circumstances.  At  15,000  volts  indications 
are  still  very  favorable,  but  at  pressures  of  about  20,000 
insulation  begins  to  involve  difficulties,  both  from  extensive 
leakage  in  bad  weather  and  occasional  puncture.  Here  porce- 
lain shows  its  marked  superiority,  and  special  insulators  of 
this  material  can  be  depended  on  to  keep  down  leakage  and 
resist  puncture  at  more  than  20,000  volts  under  ordinary  cir- 
cumstances. There  has  not  yet  been  long  enough  experience 
at  these  higher  limits  to  determine  the  probable  effects  of 
time  and  bad  weather.  The  most  striking  experiments  in  this 
line  were  carried  out  during  the  past  few  years  at  Telluride,  Col., 
under  the  direction  of  the  Westinghouse  Company  A  line 
mostly  composed  of  common  telegraph  wire,  and  about  3.5 
miles  long,  has  been  worked  successfully  for  considerable 
periods  at  pressures  up  to  55,000  volts,  transmitting  a  little 
less  than  looHP  to  a  synchronous  monophase  motor.  Within 
the  past  year  the  Telluride  experiments  have  been  extended 
to  operation  for  several  weeks  at  a  time  at  a  pressure  of  the 
enormous  magnitude  of  110,000  volts!  Various  kinds  of  in- 
sulators have  been  tried,  including  several  makes  of  porcelain 
and  glass.  The  best  results  have  been  obtained  from  highly 
vitrified  porcelain  and  frorn  special  glass  insulators.  Poor 
porcelain  appears  to  be  inferior  to  common  Western  Union 
glass  insulators.  At  55,000  volts  the  line  wires  are  luminous 
at  night  with  a  pale  blue  haze,  and  the  tie  wire  forms  a  halo 
about  each  insulator.  The  static  strain  can  be  plainly  felt  in 
riding  under  the  line,  producing  the  customary  bristling  effect 


THE  LINE.  441 

on  the  hair.  The  loss  of  energy  from  leakage  is  trifling,  how- 
ever, under  ordinary  conditions,*  and  throughout  these  ex- 
periments it  has  plainly  appeared  that  the  difficulties  of  work- 
ing at  such  voltages,  while  considerable,  are  yet  much  less 
formidable  than  had  previously  been  supposed. 

Let  us  now  sum  up  our  present  knowledge  of  the  transmis- 
sion of  electrical  energy  over  high  voltage  lines.  From  a  con- 
siderable amount  of  experience,  we  are  sure  that  there  is  no 
real  difficulty  whatever  in  establishing  and  maintaining  ade- 
quate insulation  of  either  direct  or  alternating  currents  up  to 
an  effective  pressure  of  10,000  volts.  Above  this  the  experi- 
ments are  less  numerous,  but  it  is  quite  certain  that  satis- 
factory results  can  regularly  be  reached  up  to  25,000  without 
very  extraordinary  precautions.  With  good  climatic  con- 
ditions 30,000  or  40,000  may  be  considered  entirely  practi- 
cable, with  reasonable  precautions. 

At  still  higher  voltages  the  difficulties  are  likely  to  multiply 
rapidly,  and  a  point  will  ultimately  be  reached  at  which  the 
cost  of  insulating  devices  will  overbalance  the  saving  of  copper 
due  to  increased  voltage.  This  point  is  at  present  indeter- 
minate, and  will  always  depend  on  the  amount  of  power  to  be 
transmitted,  the  permissible  loss  in  the  line  and  unknown 
variables  involving  repairs  and  depreciation,  cost  and  deprecia- 
tion of  transformers  and  so  on.  It  is  quite  impossible  from 
present  data  to  set  such  a  limit  even  approximately,  for  we 
know  as  yet  nothing  of  the  relative  difficulty  of  insulating  volt- 
ages considerably  above  the  range  of  our  experience.  The 
next  few  years  will  show  great  progress  in  this  direction.  In 
Chapter  XVII.  will  be  found  a  full  account  of  the  present 
state  of  high  voltage  transmission. 

In  cases  where  continuous  insulation  is  employed,  it  is 
for  one  of  two  purposes,  chiefly  to  prevent  interference 
with  the  circuit  by  such  accidents  as  twigs  or  wires  falling 
across  the  line,  and  either  short  circuiting  the  lines  or 
grounding  them.  Aside  from  this,  the  only  other  object  in 
insulation  is  to  lessen  the  danger  to  persons  accidentally 
touching  the  wires  and  to  prevent  the  current  straying  to 
other  circuits. 

*  About  three  per  cent,  in  most  of  these  tests. 


442  ELECTRIC   TRANSMISSION  OF  POWER. 

With  moderate  voltages  both  these  ends  can  be  reached  with 
a  fair  degree  of  success.  With  high  voltages  it  is  very  diffi- 
cult, and  in  many  cases  well-nigh  impossible. 

Nearly  all  materials  which  are  available  for  insulation 
deteriorate  to  a  very  marked  extent  when  exposed  to  the 
weather.  Those  substances  which  are  the  best  insulators,, 
such  as  porcelain,  glass,  mica,  and  the  like,  cannot  be  used 
for  continuous  insulation,  and,  in  fact,  our  best  insulators 
are  mechanically  so  bad  as  to  be  impracticable.  There 
is  a  large  class  of  insulators  complicated  in  chemical 
constitution,  but  mechanically  excellent;  these  are  the 
plastic  or  semi-plastic  substances  like  gutta-percha,  India 
rubber,  bitumen,  paraffin  and  the  like.  All  of  these  are 
subject  to  more  or  less  decomposition,  more  particularly  those 
which  are,  through  good  mechanical  qualities,  desirable  for 
insulation.  All  which  have  been  mentioned  are  sufficiently 
good  insulators  to  answer  every  practical  requirement,  if  they 
do  not  deteriorate. 

Gutta-percha  and  India  rubber  are  decidedly  the  best  of 
these;  but  gutta-percha  is  too  plastic  at  anything  excepting 
low  temperatures  to  be  mechanically  good.  Gutta-percha  fills, 
however,  an  unique  place  on  account  of  its  remarkable  ability 
to  withstand  the  action  of  salt  water,  and  it  is  the  most  reliable 
insulator  for  submarine  work.  For  overhead  work  it  is  nearly 
useless,  as  the  heat  of  the  sun  softens  it  so  as  to  endanger  its 
continuity,  and  even  a  moderate  increase  in  temperature  may 
decrease  its  specific  resistance  to  a  tenth  of  its  ordinary 
value. 

India  rubber  is,  by  all  odds,  the  best  all  around  insulator 
for  overhead  lines.  In  its  pure  state  it  deteriorates  with  very 
great  rapidity;  but  when  vulcanized  by  the  addition  of  a  small 
amount  of  sulphur,  its  chemical  character  is  so  changed  as  to 
resist  both  spontaneous  changes  and  those  due  to  the  atmos- 
phere to  a  very  considerable  extent,  without  injury  to  its  insu- 
lating properties.  It  is,  however,  costly,  and  is  eventually 
effected  by  the  weather.  To  cheapen  the  manufacture  of 
insulated  wire  a  large  variety  of  rubber  compounds  are  em- 
ployed, consisting  of  mixtures  of  rubber  with  various  other 
substances  intended  to  give  the  material  good  mechanical  and 
insulating  qualities  at  less  expense.  These  rubber  compounds 


THE  LINE.  443 

are  much  inferior  to  pure  vulcanized  rubber  in  point  of  specific         : 
resistance,  but   make   a   good   and   substantial   covering   for 
ordinary  purposes.     They   are   very  generally  employed    for 
commercial  work. 

Insulated  wires  for  overhead  work  may  be  divided  into  two 
classes.  First,  those  which  are  so  prepared  as  to  withstand 
the  weather  to  a  considerable  extent  and  to  retain  high  insu- 
lating properties  even  in  bad  weather.  Such  wires  are  usually 
covered  with  compound  fairly  rich  in  vulcanized  rubber,  com- 
monly protected  outside  with  a  braiding  of  cotton  saturated 
with  some  insulating  compound,  and  serving  to  protect  the 
main  insulation  from  mechanical  injury. 

The  second  class  of  wires  includes  those  in  which  no  solid 
insulating  material  is  used,  but  which  are  thoroughly  protected 
by  a  covering  of  fibrous  material  saturated  with  compounds  of 
rubber,  bitumen,  or  the  like.  These  wires  are  most  exten- 
sively used;  the  insulation  is  good  in  dry  weather,  and  fair 
under  most  ordinary  circumstances,  but  generally  greatly  in 
ferior  to  those  wires  which  are  given  a  coating  of  rubber. 

So  far  as  protection  of  the  wire  from  accidental  contacts  is 
concerned,  either  class  of  insulation  is  tolerably  effective  at 
moderate  voltages  until  the  covering  becomes  worn  or 
weathered  by  long  or  hard  usage. 

As  regards  danger  in  touching  such  wires,  at  moderate 
voltages  both  kinds  of  insulation  afford  a  fair  degree  of  pro- 
tection. At  high  voltages  neither  can  be  trusted,  in  spite  of 
the  apparently  high  insulation  resistance.  There  is  good 
reason  to  believe  that  any  insulation  employed  on  wires  is 
greatly  affected  by  the  strain  of  high  voltage.  Tests  made 
with  the  ordinary  Wheatstone  bridge  give  us  no  useful  inform- 
ation as  to  the  action  of  the  same  insulation  under  stresses 
of  5,000  or  10,000  volts.  Tests  made  with  pressures  ranging 
up  to  even  500  volts  show  generally  a  noticeable,  although  very 
irregular,  falling  off  in  resistance,  and  the  higher  the  voltage 
is  carried  the  more  likelihood  of  complete  breaking  down  of 
the  insulation  and  the  more  irregular  the  results. 

It  is  improbable  that  even  the  most  careful  insulation  with 
vulcanized  rubber  of  any  reasonable  thickness  would  give  a 
wire  which,  under  a  pressure  of  10,000  volts,  could  be  depended 
on  to  remove  all  danger  to  persons  from  accidental  contact. 


444  ELECTRIC    TRANSMISSION   OF  POWER. 

Even  if  entirely  safe  at  first,  it  would  be  unlikely  to  remain  so 
for  any  great  length  of  time.  So  serious  is  the  difficulty  of 
continuous  insulation  of  high  pressures,  that  it  is  probably  best 
not  to  place  dependence  upon  it;  but  either  to  fall  back  upon 
bare  wire  with  very  complete  insulation  at  the  supports,  or,  if 
insulated  wire  be  employed,  to  use  an  insulation  intended  only 
to  lessen  the  danger  of  short  circuits  from  falling  objects,  and 
to  treat  the  line,  so  far  as  personal  contact  goes,  precisely  as 
though  it  were  bare  wire. 

Information  regarding  the  insulation  of  lines,  whether  of 
bare  or  insulated  wire,  under  high  voltage,  is  very  scarce;  but 
all  such  lines  should  be  treated  at  all  times  as  if  they  were 
grounded,  in  spite  of  any  tests  of  the  insulation  that  may  have 
been  made.  Theoretically,  one  should  be  able  to  touch  a 
completely  insulated  circuit  without  danger  save  from  static 
charge;  but,  practically,  it  is  unsafe  so  to  treat  any  high 
voltage  circuit. 

The  writer  calls  to  mind  one  case  in  which  a  man  was 
instantly  killed,  while  standing  on  a  dry  concrete  floor,  by 
contact  with  a  10,000  volt  circuit.  He  probably  touched  a 
bare  portion  of  the  wire,  but  so  far  from  the  general  insulation 
of  the  circuit  saving  him,  the  current  which  he  received  was 
sufficient  to  burn  into  the  concrete  floor  the  print  of  the  nails  in 
one  of  his  shoes.  The  ordinary  tests  on  the  line  made  shortly 
afterward  showed  no  particular  ground,  nor  was  there  any 
reason  to  believe  that  one  existed  at  the  time  of  the  accident. 
Other  accidents,  under  similar  conditions,  have  occurred  with 
arc  light  circuits  of  lesser  voltage,  on  which  there  was  a  similar 
absence  of  perceptible  ground.  It  is  advisable,  therefore,  that 
all  high  voltage  circuits  should  be  treated  as  uninsulated,  so  far 
as  contact  is  concerned,  at  all  times,  and  if  insulation  tests  are 
to  be  made  upon  them  to  determine  the  resistance  to  grounds, 
these  tests  should  be  made  with,  at  least,  the  full  voltage  of  the 
circuit.  It  is  quite  as  well  not  to  place  too  much  reliance  on 
insulation  of  any  kind;  but  to  regard  a  high  voltage  electrical 
circuit  as  dangerous,  and  to  be  treated  with  the  same  respect  as 
is  due  to  other  useful,  but  dangerous,  agents,  like  high  pressure 
steam  and  dynamite,  neither  of  which  is  likely  to  be  abandoned 
on  account  of  the  danger  that  comes  from  careless  use.  The 
precautions  taken,  either  with  these  or  with  high  voltage  cur- 


THE  LINE.  445 

rents,  should  be  in  the  direction  of  preventing  such  careless- 
ness as  might  result  disastrously. 

An  electrical  circuit  should  be  so  installed  that  no  material 
risk  will  be  run  by  any  person  who  is  not  indulging  in  willful 
interference  with  the  line,  and  in  such  case,  if  an  accident 
occurs,  the  victim  is  deserving  of  no  more  sympathy  than  one 
who  deliberately  stands  in  front  of  an  express  train. 

If  the  circuit  is  of  bare  wire,  there  can  be  no  doubt  in  the 
mind  of  anyone  as  to  its  dangerous  character,  whereas,  if 
insulated  wire  is  employed,  there  is  likely  to  be  established  a 
certain  false  sense  of  security.  There  is  no  good  reason, 
therefore,  for  advising  the  extensive  use  of  insulated  wire  for 
high  voltage  lines. 

The  ideal  overhead  circuit  is  one  in  which  the  conductor  is 
thoroughly  insulated  as  regards  leakage,  carefully  protected 
from  danger  of  wires  or  branches  falling  across  it,  and  placed 
out  of  the  reach  of  anything  except  deliberate  interference  of 
human  beings.  There  may  be  places  at  various  points  along 
the  line  where  insulation  would  be  desirable,  in  order  to  avoid 
extensive  cutting  away  of  trees,  branches  of  which  might  fall 
upon  the  line,  or  where  local  regulations  require  the  use  of 
insulated  wire.  Except  under  these  circumstances  continuous 
insulation  increases  the  cost  and  maintenance  of  the  line 
without  giving  any  adequate  returns  in  security.  On  rare 
occasions,  portions  of  the  high  voltage  circuit  may  have  to  be 
placed  underground.  Here  only  the  very  best  quality  of 
insulation  should  be  employed,  preferably  high  grade  rubber- 
covered  cable  thoroughly  protected  by  an  outside  sheathing 
of  lead  against  the  effects  of  moisture,  and  installed  in  smooth, 
clean,  dry,  and  accessible  conduits.  The  character  and 
amount  of  the  precautions  to  be  taken  depend  on  the  voltage 
of  the  circuit,  and  there  has  not  yet  been  enough  experience, 
at  pressures  above  those  which  must  be  classified  as  moderate 
(that  is,  from  1,000  to  3,000  volts),  to  get  sufficient  data 
to  enable  one  to  say  absolutely  what  are  sufficient  precau- 
tions. The  best  course  to  follow  is,  knowing  the  voltage  at 
which  the  transmission  is  to  be  made,  to  select  a  cable  which 
cannot  be  experimentally  broken  down  by  twice  the  voltage  in 
question,  and  to  install  it  with  every  precaution  against 
deterioration.  If  any  great  work  is  to  be  accomplished, 


446  ELECTRIC    TRANSMISSION  OF  POWER. 

the  necessary  expense  is  so  considerable  as  to  make  it  well 
worth  while  to  enter  into  a  careful  investigation  as  to  the  par- 
ticular voltage  in  hand. 

Wherever  underground  cables  are  employed,  they  should  be 
subjected  to  a  daily  insulation  test,  and  the  greatest  care 
should  be  taken  with  the  joints,  which  are,  in  almost  every 
case,  the  weakest  point  in  the  insulation. 

From  what  has  been  said,  it  should  be  understood  that  while 
the  problem  of  installing  high  voltage  lines  is  unquestionably 
a  difficult  one,  we  have  not  yet  had  sufficient  experience  to  be 
able  to  say  definitely  how  difficult  it  may  be.  It  is  very  cer- 
tain that  much  more  can  be  done  than  has  been  accomplished. 
It  seems  probable  that  so  far  as  overhead  work  is  concerned, 
it  will  be  practical  to  employ  voltages  considerably  greater 
than  those  now  in  use.  Before  any  limit  can  be  set  to  the 
progress  in  this  direction,  we  need  ample  experimental  data, 
not  only  on  the  behavior  of  insulation  at  a  very  high  pressure, 
but  on  the  maximum  voltage  which  is  likely  to  be  encountered 
when  a  certain  effective  voltage  is  to  be  employed.  This 
opens  up  a  wide  field  for  investigation,  involving  conditions 
of  unknown  seriousness,  connected  especially  with  the  elec- 
trical peculiarities  of  alternating  currents,  which  there  is 
every  reason  to  believe  will  be  employed  almost  exclusively  on 
high  voltage  work. 

The  special  difficulties  to  be  met  in  working  with  alternating 
currents  are  two — inductance  in  the  line  and  apparatus,  and 
electrostatic  capacity,  accompanied  by  the  very  serious  phe- 
nomena of*electrical  resonance.  In  addition  to  these,  what- 
ever the  character  of  the  current  used  for  transmission  purposes, 
there  is  danger  of  getting  accidentally  upon  the  line  a  voltage 
much  higher  than  the  normal.  Inductance  is  met  with  to  a 
very  considerable  extent  in  all  alternating  circuits;  resonance 
in  a  small  degree  is  probably  much  commoner  than  is  generally 
supposed,  and  abnormal  voltage,  due  to  the  generators  them- 
selves, must  always  be  guarded  against. 

Passing  at  once  to  the  practical  side  of  the  question,  we 
find  that  when  an  alternating  current  is  sent  through  any 
conductor,  it  has  to  deal  not  only  with  the  electrical  resist- 
ance of  that  wire,  but  with  a  virtual  resistance  due  to  the 
fact  that  the  electromagnetic  stresses  set  up  at  any  point 
of  the  conductor  set  up  electromotive  forces  at  other  points 


THE  LINE.  447 

in  the  same  conductor,  which  oppose  and  retard  the  passage 
of  the  current. 

These  matters  have  been  fully  discussed  theoretically  in 
Chapter  IV.,  and  hence  will  be  here  but  briefly  mentioned. 

For  example,  if  a  wire  be  bent  into  a  couple  of  spiral  coils 
like  Fig.  219,  the  electromagnetic  field  of  one  coil  will  affect  the 


FIGS.    2IQ    AND    220. 

other,  just  as  we  have  induction  from  one  separate  ring* to 
another  in  Fig.  4,  page  12.  If  such  a  spiral  has  an  iron  core, 
this  self-inductance  will  be  much  increased.  Even  if  only  a 
straight  wire  be  concerned  in  the  carrying  of  current,  there 
will  be  a  similar  inductive  relation  between  the  inner  and  outer 
portions  of  the  wire  at  any  point,  since  the  electromagnetic 
stresses  exist  inside  the  wire  as  well  as  outside. 

Let  Fig.  220  represent  a  circuit  carrying  an  alternating  cur- 
rent, which  at  a  given  moment  is  flowing  as  shown  by  the 
arrows.  The  electromagnetic  field  set  up  by  this  current  in 
the  loop  has  a  direction  perpendicular  to  the  plane  of  the 
paper,  and  sets  up  an  E.  M.  F.  opposing  that  of  the  wire.  The 
greater  the  area  of  the  loop,  i.  ^.,  the  farther  apart  the  two 
wires,  the  greater  proportion  of  the  electromagnetic  field  will 
pass  within  the  loop  and  produce  self-induction. 

Similarly,  the  larger  the  wires  for  a  given  distance  between 
them,  the  less  effective  field  within  the  loop  to  set  up  induc- 
tance. In  fact,  the  amount  of  inductance  in  the  circuit 
depends  directly  on  the  ratio  between  the  radii  of  the  wires 
and  the  distance  between  them.  So  if  the  diameter  of  the 
wire  is  decreased  to  one-half  the  original  amount,  the  wires 
must  be  strung  only  half  as  far  apart  in  order  to  produce  the 
same  inductance. 

The  practical  effect  of  inductance  in  the  line  is  to  neces- 
sitate the  use  of  an  initial  E.  M.  F.  large  enough  to  overcome 
the  inductive  loss  of  voltage,  as  well  as  that  due  to  resistance, 
and  so  keep  the  E.  M.  F.  at  the  receiving  end  of  the  line  up 


448 


ELECTRIC   TRANSMISSION  OF  POWER. 


to  its  proper  value.  The  simplest  way  to  take  account  of 
inductance  in  figuring  a  line  is  to  treat  it  as  an  additional 
resistance,  which,  while  it  does  not  materially  increase  the 
energy  lost  in  the  line,  must  be  taken  into  one's  calculation 
of  the  E.  M.  F.  to  be  delivered  by  the  generator.  Its  effect 
is  to  require  a  little  greater  generator  capacity,  /.  e.t  a  machine 
giving,  say,  1,050  volts,  must  be  used  when  the  effective  volt- 
age wanted  is  only  1,000.  The  combined  resistance  and 
inductance  of  a  line  is  called  its  impedance,  and  this  may  be 
readily  tabulated  for  common  cases,  giving  the  impedance 
factor — that  is,  the  relation  of  the  total  impedance  to  the  real 
line  resistance.  This  is  the  easiest  way  of  considering  induc- 
tance in  the  line  computation. 

The  following  table  gives  the  impedance  factors  for  wires 
from  oooo  to  6  B.  &  S.,  for  120  and  60  cycles  per  second,  and  for 
a  uniform  distance  between  wires  of  18  inches.  The  imped- 
ance factor  increases  with  the  size  of  wire  because  the  resist- 
ance decreases  rapidly  for  larger  wires;  and  the  question  of 
length  of  wire  does  not  enter,  because  both  inductance  and 
resistance  increase  directly  with  the  length,  so  that  they  are 
always  proportional.  The  E.  M.  F.  of  inductance  increases 
with  the  cycles  since  the  field  in  the  loop  changes  more  rap- 
idly, and,  therefore,  gives  a  greater  induced  E.  M.  F. 


IMPEDANCE  FACTOR. 

.NO.   WIRES. 

60  PERIODS. 

120  PERIODS. 

OOOO 

2.3 

4-3 

ooo 

2.0 

3-7 

00 

•79 

308 

0 

•  53 

2-54 

I 

•39 

2.16 

2 

.28 

.85 

3 

.19 

.61 

4 

.12 

1.42 

5 

.08 

.30 

6 

•05 

.21 

The  figures  in  the  table  are  for  a  complete  metallic  circuit, 
consisting  of  two  parallel  copper  wires,  and  the  impedance 
factors  may  be  applied  at  once  either  to  determine  the  virtual 


THE  LINE.  449 

resistance  of  a  given  wire,  or  the  total  drop  of  voltage  that 
will  take  place  on  a  given  circuit.  Suppose,  for  example,  that 
we  have  an  alternating  circuit  composed  of  No.  i  wire, 
delivering  1,000  volts  at  the  receiving  station.  Suppose  the 
volts  lost  due  to  resistance  alone  at  full  load  current  are  90, 
and  the  plant  is  working  at  60  cycles.  Then  the  total  loss 
in  volts  will  be  90  x  i-39  =  I25-1,  and  the  voltage  of  the 
generator  must  be  1,125.1  in  order  to  give  1,000  volts  at  the 
end  of  the  line;  and  so  on  for  any  load  on  any  size  of  wire. 
It  is  apparent  that  it  is  undesirable  to  use  very  large  wires  for 
high  frequency  alternating  currents,  and  in  some  cases  it  is 
better  to  divide  the  circuit,  using  two  No.  i  wires,  for 
instance,  instead  of  one  No.  ooo. 

It  must  not  be  forgotten  that  one  of  the  effects  of  induc- 
tance is  to  cause  the  current  to  lag  behind  the  E.  M.  F.,  so 
that  for  the  delivery  of  a  given  amount  of  energy  at  the 
receiving  end,  a  current  larger  than  the  watts  divided  by 
the  E.  M.  F.  must  be  delivered  by  the  generator,  which  must 
be  of  corresponding  capacity.  This  increase  may  amount  to 
10  or  15  per  cent.,  according  to  the  character  of  the  load, 
often  less,  and  sometimes  more. 

Intimately  connected  with  this  is  the  fact  that  in  finding 
the  voltage  at  any  part  of  the  system  account  must  be  taken 
of  all  the  impedances  in  circuit,  for  they  are  not  generally  in 
the  same  phase,  so  that  their  geometrical  sum  is  almost  always 
less  than  their  algebraic  sum.  Hence,  while  the  impedance 
factor  applied  to  the  line  will  give  correctly,  neglecting  line 
capacity,  the  drop  produced  in  each  line  wire,  the  voltage 
across  the  terminals  of  the  line  will  usually  be  different  from 
that  figuied  from  the  impedance  factor  applied  to  the  line 
alone.  This  difference  is,  generally,  not  great  unless  there 
is  capacity  at  the  receiving  end.  All  these  considerations 
complicate  the  exact  computation  of  a  system,  and  unless 
its  character  is  thoroughly  predetermined,  no  exact  computa- 
tion can  be  made. 

Practically  these  complications  can  be  for  the  most  part 
brushed  aside  in  designing  a  plant.  In  very  few  cases  will  the 
impedance  factor  of  a  line  be  over  2,  which  means  generally 
that  the  whole  matter  can  be  take  care  of  by  an  increase  in 
generator  voltage  of  less  than  10  per  cent. 


45°  ELECTRIC   TRANSMISSION   OF  POWER. 

It  should  be  noted  that  the  impedance  factors  given  can  be 
applied  at  once  to  three-phase  circuits,  in  which  the  wires  are 
equidistant.  The  impedance  factor  of  such  a  system  is  the 
same  as  that  of  any  pair  of  the  wires.  Really  the  most  serious 
practical  difficulties  in  an  ordinary  alternating  plant  are  those 
in  which  the  generator  is  involved  by  inductances  in  the 
system.  These  are  of  far  greater  moment  than  the  impedance 
factor  of  the  line.  An  inductance  in  the  system  produces  two 
effects  on  the  generator — first,  as  just  noted,  it  demands  a 
larger  current  to  deliver  the  same  energy;  second,  it  tends 
to  beat  down  the  E.  M.  F.  of  the  machine.  This  effect  is 
analogous  to  that  produced  by  shifting  the  brushes  of  a 
continuous  current  generator  away  from  the  position  of 
maximum  E.  M.  F.  (See  Chapter  V.) 

This  reaction  of  the  armature  is  serious  in  that  it  not  only 
demands  a  considerable  increase  in  the  exciting  current,  but 
causes  a  severe  strain  on  the  insulation  when  it  suddenly 
ceases.  It  is  not  uncommon  to  find  an  alternator  that  requires 
on  a  heavy  inductive  load  double  the  light-load  excitation  of 
the  field.  For  instance,  if  the  voltage  be  2,000  on  open  cir- 
cuit, the  excitation  may  have  to  be  increased  on  inductive 
load  to  a  point  that  on  open  circuit  would  give  4,000  volts. 
If,  now,  this  load  is  cut  off,  or  the  line  is  broken,  the  insula- 
tion will  be  exposed,  momentarily,  at  least,  to  double  the 
normal  voltage. 

Such  generators  should  not  be  used  on  inductive  loads  or 
in  any  case  where  the  extra  strain  on  the  insulation  is  impor- 
tant. It  is  perfectly  easy  to  build  a  generator  which  requires 
only  10  to  25  per  cent,  more  excitation  at  full  and  inductive 
load  than  at  no  load,  and  such  machines  should  be  used  in  all 
cases  where  a  steady  voltage  under  all  working  conditions  is 
needed.  The  other  type  has  its  uses,  but  the  general  work  of 
power  transmission  is  not  one  of  them.  With  a  properly 
designed  machine,  inductive  load  is  little  to  be  feared. 

Another  possible  source  of  danger  is  that  under  certain 
conditions  of  inductive  load  the  reaction  of  the  load  on  the 
generator,  without  materially  lowering  its  effective  voltage, 
may  so  change  the  shape  of  the  E.  M.  F.  wave  as  to  give  to  it 
an  abnormally  high  maximum,  and  thereby  greatly  to  increase 
the  strain  on  the  insulation.  This  effect  may  readily  occur, 


THE  LINE.  451 

but  usually  in  so  small  a  degree  as  to  be  of  little  moment. 
Occasionally,  owing  to  a  combination  of  severe  inductive  load 
and  badly  designed  generator,  the  results  may  be  somewhat 
formidable,  the  more  so  as  the  change  takes  place  under 
heavy  load  and  not,  as  in  the  case  just  treated,  only  on  open 
circuit  or  a  sudden  light  load.  The  rise  in  pressure  thus  pro- 
duced may  amount  to  several  times  the  nominal  voltage.  The 
same  sound  principles  of  design  that  insure  good  regulation 
under  changes  of  load  will  obviate  any  danger  of  this  kind. 
In  fact,  most  of  the  possible  disturbing  factors  in  alternating 
current  work  become  negligible  in  an  installation  carried  out 
with  regard  for  the  general  principles  of  good  engineering. 

These  abnormalities  of  voltage  lead  naturally  to  the  con- 
sideration of  another  far  more  serious,  due  to  the  static 
capacity  of  the  system.  Of  course,  the  fact  that  under  cer- 
tain circumstances  capacity  in  the  system  will  cause  a  lessen- 
ing of  the  apparent  "drop"  on  the  line,  or  even  overcome  it 
altogether  and  show  a  higher  voltage  at  the  receiving  end  than 
at  the  generator,  is  already  well  known  to  the  reader.  This 
is,  however,  but  a  special,  and,  save  for  its  convenience  in 
connection  with  synchronous  motors,  a  trivial  case  of  the 
phenomenon  known  as  electrical  resonance.  (See  p.  150.) 

The  practical  importance  of  this  has  been  but  recently 
realized,  although  the  thing  itself  is  strictly  analogous  to  very 
familiar  occurrences.  Suspend  a  heavy  weight  by  a  string 
a  yard  long  or  so,  and  then  begin  tapping  the  weight  lightly 
with  the  finger.  So  long  as  the  taps  are  irregular  or  bear  no 
particular  relation  to  the  time  of  oscillation  of  the  weight 
considered  as  a  pendulum,  the  only  effect  will  be  slightly 
jerky  vibrations.  As  soon,  however,  as  the  taps  are  so  timed 
as  to  coincide  with  the  swinging  period  of  the  weight,  it  will 
slowly  get  under  way,  and  presently  work  up  an  oscillation  of 
considerable  amplitude.  Each  tap  catches  the  weight  at  the 
end  of  the  double  swing  started  by  the  previous  tap  and 
augments  its  motion. 

Every  system  capable  of  oscillation  is  similarly  affected  by 
impulses  coinciding  with  its  natural  period,  which  is  deter- 
mined by  its  physical  properties.  This  rapid  growth  of  am- 
plitude through  synchronous  impulses  is  generally  known  as 
resonance  from  the  very  marked  way  in  which  the  phenom- 


452  ELECTRIC   TRANSMISSION  OF  POWER. 

enon  appears  in  a  sounding  system  such  as  that  composed  of 
a  tuning  fork  and  the  box  (resonator)  to  which  it  is  generally 
affixed. 

Every  electrical  system  has  a  definite  period  of  oscillation 
determined  by  its  particular  properties.  If  we  could  apply  an 
instantaneous  electromotive  stress  to  any  point  of  it,  the  effect 
would  be  that  the  resulting  strain  would  travel  back  and  forth 
with  a  definite  frequency  until  its  energy  would  be  completely 
exhausted  by  doing  work  on  various  parts  of  the  system.  The 
action  resembles  that  which  takes  place  when  we  strike  the 
end  of  a  long  rod  with  a  hammer.  An  impulse  is  sent  out  at 
a  rate  depending  on  elasticity,  density,  and  so  forth,  travels.  to 
the  end  of  the  rod,  is  reflected,  and  so  goes  on  swinging  back 
and  forth  until  the  energy  is  frittered  away.  This  corresponds 
to  electric  oscillations  on  open  circuit. 

The  two  properties  of  an  electrical  system  which  determine 
its  vibration  period  are  its  self-induction,  which  is  analogous 
to  inertia,  and  its  capacity,  which  resembles  elasticity  in  the 
dielectric,  capable  of  taking  up  and  returning  energy.  Resist- 
ance, like  intermolecular  friction  in  the  rod  just  referred  to, 
determines  the  rate  at  which  the  vibrations  will  die  out  by 
yielding  up  their  energy  to  the  system,  but  has  ordinarily 
a  negligible  effect  on  the  vibration  period. 

This  period  in  an  electric  circuit  is  to  a  close  degree  of 
approximation  given  by  the  following  formula: 

T  =  .00629  ^TC  = 


In  this  T  is  the  natural  time  period  of  the  circuit  expressed 
in  seconds,  L  is  the  coefficient  of  self-induction  in  henrys, 


2  MICROFARADS 
1  HENRY 


FIG.  221. 


and  C  the  capacity  in  microfarads.  For  example,  suppose  we 
are  dealing  with  a  circuit  of  which  the  capacity  is  two  micro- 
farads and  the  self-induction  one  henry.  Let  it  be  arranged 
as  in  Fig.  221.  For  simplicity  the  inductance  and  capacity  are 
shown  localized  and  in  series  as  would  happen  if  a  line  ran 


THE  LINE.  453 

through  a  group  of  series  transformers  and  thence  into  a  cable. 
If  the  line  were  open-circuited  beyond  the  cable,  we  might  find 
a  very  severe  strain  on  the  cable  insulation.  The  period  of 
this  line  would  be  .00887  second — about  113  cycles  per  second. 
If  this  should  chance  to  be  the  frequency  of  the  generator  it 
would  be  in  resonance  with  the  line,  and  each  wave  of  E.  M.  F. 
sent  out  by  the  generator  would  add  itself  to  another  wave  just 
starting  out  in  the  same  direction.  A  period  later  these  two 
added  E.  M.  Fs.  would  be  reinforced  by  the  next  generator 
wave,  and  so  on  indefinitely. 

The  only  thing  which  prevents  the  resultant  voltage  from 
rising  indefinitely  is  the  effect  of  energy  losses  in  causing  each 
wave  to  die  out  gradually  as  it  continues  its  oscillations,  so  that 
only  a  limited  number  of  waves  can  add  materially  to  the 
resultant  E.  M.  F.  across  the  terminals  of  the  capacity. 

In  a  given  circuit  the  relation  between  the  initial  volta»ge 
and  the  voltage  of  resonance  can  be  easily  determined  to 
a  fair  degree  of  approximation.  It  is,  neglecting  minor 
reactions,  as  we  have  alread 


In  this  equation  E  is  the  E.  M.  F.  of  resonance,  n  the  fre- 
quency, L  the  self-induction  in  henrys,  R  the  ohmic  resist- 
ance, and  E  the  initial  voltage.  Applying  this  formula  to  the 
case  just  discussed,  and  assuming  the  resistance  of  the  line 
to  be  15  ohms  and  the  initial  voltage  to  be  2,000,  we  find 

E'  —  -  =  15,066  volts.     A  very  moderate  line 

voltage  may  thus,  in  a  resonant  line,  give  rise  to  a  pressure 
quite  capable  of  rupturing  any  ordinary  cable,  or  causing  serious 
trouble  on  an  overhead  line,  to  say  nothing  of  greatly  increas- 
ing the  danger  to  persons  and  property.  If  the  working 
pressure  were  10,000  or  15,000  volts,  the  E.  M.  F.  of  resonance 
might  rise  to  an  appalling  amount. 

Fortunately  this  theoretical  value  is  in  practice  generally 
much  reduced  by  hysteretic  losses  and  Foucault  currents  in 
any  iron-cored  coils  in  circuit,  waste  of  energy  in  the  dielectric, 
and  other  minor  causes  of  damping  the  electrical  oscillations, 


454  ELECTRIC   TRANSMISSION  OF  POWER. 

even  when  resonance  is  complete.  Still,  dangerous  rises  in 
voltage  are  very  possible.  When  the  frequency  of  the  applied 
E.  M.  F.  differs  somewhat  from  the  natural  period  of  the  line, 
resonant  effects  can  evidently  still  take  place,  but  in  a  rapidly 
lessening  degree;  when  the  oscillations  are  strongly  damped 
by  the  presence  of  iron,  the  total  resonant  rise  is  considerably 
diminished,  but  it  varies  less  rapidly  as  the  resonant  frequency 
is  departed  from. 

A  resonance  curve  for  various  capacities  shows  that  the 
rise  of  voltage  extends  over  quite  a  wide  range  of  variation 
of  capacity,  but  is  large  over  but  a  small  range.  The  shape 
of  such  a  curve  necessarily  varies  widely,  as  the  resonance  is 
more  or  less  damped  by  resistance,  iron-cored  coils  and  so 
forth;  but  we  may  be  quite  sure  that  the  maximum  resonance 
will  occur  at  not  far  from  the  point  indicated  by  our  equa- 
tion for  the  vibration  period  of  the  circuit,  and  that  the  maxi- 
mum E.  M.  F.  of  resonance  will  usually  be  considerably  less 
than  that  given  by  the  theoretical  equation. 

In  practical  alternating  circuits  the  current  wave  is  never 
truly  sinusoidal,  but  consists  of  a  main  or  fundamental  wave 
with  the  odd  (/.  <?.,  3d,  5th,  yth,  etc.)  harmonics  of  various 
amplitudes  superimposed  upon  it.  In  nearly  every  case  the 
third  harmonic  is  the  most  prominent  and  is  quite  capable  of 
causing  resonance,  even  to  a  dangerous  degree,  if  it  happens  to 
fall  in  with  the  frequency  of  the  system.  The  extent  of  the 
rise  in  voltage,  however,  is  very  much  less  than  that  produced 
by  resonance  with  the  fundamental  frequency.  The  point  at 
which  resonance  occurs  and  the  rise  of  E.  M.  F.  are  found  for 
the  harmonics  by  the  formulae  already  given. 

So  far  as  the  line  is  concerned,  the  facts  regarding  resonance 
can  be  easily  computed  with  tolerable  accuracy.  From  well 
established  data  it  is  evident  that  the  line  capacities  are  gener- 
ally so  small  as  to  make  the  oscillation  period  so  short  as  not 
to  correspond  with  the  frequencies  in  ordinary  use  except  in 
the  upper  harmonics,  which  are  generally  of  small  moment, 
although  one  case  of  severe  resonance  from  a  higher  har- 
monic (probably  the  yth)  has  come  to  the  author's  notice. 

To  facilitate  computation  the  following  table  is  given,  show- 
ing the  inductance  and  capacity  of  ordinary  overhead  lines  per 
mile  in  millihenrys  and  microfarads  respectively.  The  wires 


THE  LINE. 


455 


are  supposed  to  be  of  copper  and  to  be  suspended  as  a  parallel 
pair  at  a  distance  of  18  inches,  as  in  the  case  of  the  table  of 
impedance  factors.  Within  the  range  of  ordinary  practice 
varying  the  height  above  the  ground  makes  no  sensible  differ- 
ence in  inductance  or  capacity. 


SIZE  No. 

INDUCTANCE. 

CAPACITY. 

oooo 

.48 

.0102 

000 

•  52 

.0099 

00 

.56 

.0097 

o 

•59 

.0095 

I 

.63 

.0093 

2 

.67 

.0090 

c        3 

•71 

.0088 

4 

•  74 

.0086 

5 

.78 

.0084 

6 

.82 

.0083 

It  must  be  remembered  that  not  only  the  line  capacity,  but 
the  capacity  of  the  receiving  apparatus,  must  be  considered. 
The  former  is  but  small,  except  in  the  case  of  underground  Or 
submarine  cables,  for  which  the  capacities  are  likely  to  be 
from  ^  to  ^  microfarad  per  mile,  as  ordinarily  manufactured. 
High  voltage  translating  devices,  like  synchronous  motors  and 
transformers,  often  may  have  static  capacities  of  several  tenths 
of  a  microfarad. 

As  a  matter  of  fact,  experience  seems  to  show  that  one  is 
not  likely  to  stumble  upon  very  serious  resonance  in  overhead 
lines,  although  in  cables  it  is  easily  possible.  On  the  other 
hand,  it  is  more  than  likely  that  resonance  of  a  minor  kind, 
mostly  from  harmonics,  is  far  commoner  than  is  generally  sup- 
posed. High  pressure  alternating  currents  often  show  a  tend- 
ency to  jump  across  air  spaces  and  insulation  that  is  hard  to 
account  for  otherwise.  This  tendency  must  not  be  confused 
with  the  occasional  displays  of  atmospheric  electricity  on  over- 
head lines.  It  is  not  uncommon  to  get  somewhat  severe 
shocks  from  an  aerial  line  entirely  disconnected  from  any 
machinery. 

The  most  obvious  way  of  avoiding  trouble  from  resonance, 
when  it  exists  or  is  threatened,  is  to  vary  the  frequency,  induct- 
ance or  capacity  so  as  to  throw  the  oscillation  period  of  the 
system  and  the  frequency  of  the  current  far  apart.  The  fre- 


456  ELECTRIC   TRANSMISSION  OF  POWER. 

quency  is  usually  fixed  by  other  considerations,  but  inductance 
and  capacity  are  varied  with  the  utmost  ease.  In  fact,  they 
are  never  constant  in  practice. 

The  readiest  way  of  getting  a  clear  idea  of  the  inductance 
of  a  system  is  from  its  power  factor,  *.  e.,  the  ratio  between 
the  apparent  energy  as  found  from  voltmeter  and  ammeter 
readings  and  the  real  energy  as  given  by  a  wattmeter.  This 
not  only  gives  the  current  which  must  be  carried  for  a  given 
amount  of  energy,  but  measures  the  angle  of  lag  between  cur- 
rent and  E.  M.  F.  In  fact, 

Energy_  =  cos  , 
Volt-amperes 

which  gives  the  angle  of  lag  at  once.     But  in  general 
Tan  *  =  2  *     L. 


So  that  knowing  the  ohmic  resistance  of  a  circuit  and  its 
"power  factor,"  we  can  get  a  tolerable  approximation  to  its 
coefficient  of  self-induction,  R  being  reckoned  in  ohms  and 
L  in  henrys.  As,  however,  the  lag  is  influenced  by  both 
inductance  and  capacity,  the  formula  is  principally  of  use  in 
connection  with  individual  pieces  of  apparatus. 

From  a  practical  standpoint  it  is  quite  out  of  the  question  to 
compute  the  total  inductance  of  a  proposed  system  on  which 
any  resonance  effects  depend,  with  anything  like  accuracy, 
since  the  character  and  amount  of  the  apparatus  is  indetermi- 
nate. One  can,  however,  gain  from  experience  a  tolerable 
idea  of  the  probable  lag  in  a  mixed  system,  which  will  at  least 
tell  whether  there  is  imminent  danger  of  resonance.  There  is 
really  little  danger  save  from  harmonics  at  ordinary  frequen- 
cies unless  the  static  capacity  involved  is  large  —  several  micro- 
farads at  least.  ' 

One  other  phenomenon  in  connection  with  alternating  cur- 
rent work  deserves  notice  here.  Several  years  ago  Lord 
Kelvin  pointed  out  that  the  ohmic  resistance  of  a  copper  wire 
was  greater  for  alternating  than  for  direct  currents.  This  is 
for  the  reason  that  the  current  density  through  the  cross 
section  of  the  conductor  is  not  uniform  when  the  current  is  an 
alternating  one.  The  instantaneous  propagation  of  any  current 


THE  LINE.  457 

is  mainly  along  the  surface  of  the  conductor,  and  only  after  a 
measurable,  though  short,  time  is  the  condition  of  steady  flow 
reached. 

When  the  current  rapidly  alternates  in  direction  the  interior 
of  the  conductor  is  thus  comparatively  unutilized,  for  before 
the  flow  has  settled  into  uniformity  its  direction  is  changed, 
and  the  original  surface  flow  is  resumed.  The  larger  the  wire 
and  the  greater  the  frequency  the  more  marked  this  effect. 
Fortunately,  with  the  common  sizes  of  wire  and  the  frequencies 
ordinarily  employed  for  power  transmission  work,  it  is  quite 
negligible.  At  60  periods  the  increase  of  resistance  due  to 
this  cause,  in  a  conductor  even  half  an  inch  in  diameter,  is  less 
than  one-half  of  i  per  cent.  Any  line  wire  that  is  allowable 
on  the  score  of  its  impedance  factor  will  be  unobjectionable 
on  this  account  as  well.  Only  occasionally,  as  in  bus  bars  for 
low  voltage  switchboards,  is  it  worth  considering,  and  in  such 
cases  the  use  of  flat  bars,  half  an  inch  or  less  thick,  or  tubular 
conductors,  will  obviate  the  difficulty. 

We  have  now  investigated  all  the  important  factors  that 
enter  into  the  design  of  a  transmission  line,  whether  for  direct 
or  alternating  currents.  Let  us  review  them  with  the  idea  of 
seeing  how  they  enter  into  practical  cases.  First  comes  the 
all-important  question  of  initial  voltage,  involving  the  choice 
between  the  direct  generation  of  the  working  pressure  or  its 
derivation  from  transformers,  if  alternating  currents  are  used. 
We  have  already  seen  the  practical  limitations  of  voltage  for 
direct  currents.  With  alternators  the  commutator  troubles 
are  absent,  and  the  limitations  are  those  imposed  by  generator 
design.  The  higher  the  voltage  of  a  dynamo  the  more  space 
on  the  armature  must  be  allowed  for  insulation,  thereby  cut- 
ting down  the  output  of  the  machine.  Hence  the  practicable 
voltage  depends  on  the  size  of  the  generator. 

In  a  general  discussion  it  is  difficult  to  make  exact  state- 
ments as  to  what  can  or  cannot  be  done,  but  experience  seems 
to  show  that  at  present  10,000  to  13,000  volts  are  the 
greatest  pressures  that  can  economically  be  derived  from  the 
generator,  even  in  very  large  units,  while  in  units  of  :oo  or 
200  KW  it  is  selJom  advisable  to  go  above  3,000  to  5,000. 
Higher  voltage  than  this  has  been  attempted,  but  there  is 
good  reason  to  believe  that  except  in  large  machines  the  re- 


458  ELECTRIC    TRANSMISSION  OF  POWER. 

duced  output  and  increased  cost  of  construction  lead  to  a 
higher  cost  per  KW  output  than  if  low  voltage  generators 
with  transformers  were  employed. 

For  present  purposes,  then,  we  may  assume  that  in  a  large 
number  of  cases  the  voltage  of  the  generator  will  not  be 
materially  above  3,000  volts.  With  raising  transformers  10,000 
or  20,000  volts  is  known  to  be  a  practicable  pressure.  At 
some  particular  distance,  then,  in  each  case,  the  annual 
interest  and  depreciation  charge  against,  say  a  3,000  volt  line, 
will  equal  the  interest  and  depreciation  charge, against,  say  a 
10,000  volt  line,  and  its  raising  and  reducing  transformers. 
For  a  fair  comparison  two  things  must  be  noted — first,  that 
3,000  volts  is  an  available  pressure  on  a  working  circuit,  so 
that  it  has  the  advantage  over  10,000  volts,  of  both  sets  of 
transformers — second,  that  the  systems  must  be  of  the  same 
efficiency:  *.  e.,  the  line  loss  in  the  first  case  should  be  equal 
to  the  losses  in  both  line  and  transformers  in  the  second  case. 

The  exact  distance  at  which  the  costs  by  the  two  methods 
become  equal  must  be  computed  from  the  data  above  outlined 
in  each  individual  case  on  account  of  the  variations  introduced 
by  size  of  units,  cost  of  copper  and  transformers  delivered, 
differing  conditions  of  depreciation,  and  so  on.  The  distance, 
however,  is  generally  longer  than  would  be  at  first  sight  sup- 
posed. Assuming  an  equal  rate  of  interest  and  depreciation 
in  each  case,  a  few  hundred  HP  as  the  amount  transmitted 
and  current  prices  of  copper  and  large  transformers,  we 
arrive  at  the  following  approximate  result:  At  about  6  miles 
3,000  volts  from  the  generator  and  10,000  volts  with  raising 
and  reducing  transformers  become  equal  in  cost.  This  dis- 
tance rises  to  between  9  and  10  miles  if  5,000  volts  only  are 
derived  from  the  transformers.  Generators  wound  for  above 
3,000  volts  generally  will  require  reducing  transformers  for 
the  working  circuit,  so  that  they  have  the  advantage  of  the 
raising  and  reducing  system  by  only  one  set  of  transformers. 

In  most  cases  it  is  desirable  either  to  generate  a  voltage 
which  can  be  used  on  the  working  circuit  or  to  use  from  10,000 
to  20,000  volts  derived  from  transformers  or  in  large  units 
from  the  generator  itself.  So  much  for  the  selection  of  a 
proper  line  voltage. 

As  to  loss  in  the  line  much  has  been  said  already,  and  the 


THE  LIKE.  459 

best  advice  that  can  be  given  is  to  make  a  few  trial  computa- 
tions along  the  general  lines  indicated.  Almost  every  case 
will  require  special  treatment  in  certain  particulars,  depending 
on  the  conditions  of  service.  For  example,  a  common  com- 
plication is  the  supply  of  power  or  light,  or  both,  at  a  point 
perhaps  halfway  along  the  line.  Then,  according  to  the 
amount  and  kind  of  service,  it  may  be  desirable  simply  to  tap 
the  line,  to  tap  the  line  for  power  and  use  a  motor  generator 
for  lights,  to  establish  a  substation  with  regulating  apparatus, 
to  compound  the  generator  for  the  point  in  question  and  use 
either  of  the  above  methods  at  the  end  of  the  line,  to  install 
rotary  transformers,  or  to  run  a  separate  line  with  regulators 
at  the  generating  station.  Such  details  will  be  treated  at 
length  in  Chapter  XIV. 

The  line  structure  should  preferably  be  of  bare  copper  wire 
carried  on  strong  wooden  poles.  Do  not  put  it  underground 
unless  you  have  to  do  so.  It  may  be  necessary  to  insulate 
portions  of  the  wire,  but  it  is  best  not  to  put  much  faith  in 
an  insulating  covering.  Instead,  it  is  desirable  to  make  a 
Very  thorough  job  of  insulation  at  the  supports,  and  provide 
for  the  easy  inspection  of  the  line. 

In  using  alternating  currents  inductance  in  the  line  must 
always  be  considered.  Practically  it  means  raising  the  voltage 
of  the  generator  or  raising  transformers  by  a  few  percent., 
unless  a  fair  part  of  the  load  is  in  synchronous  motors  which 
can  be  employed  to  counteract  the  inductive  drop.  In  nearly 
every  case  its  real  importance  is  small,  in  spite  of  its  scaring 
the  uninitiated  now  and  then. 

So,  too,  with  the  inductive  load.  Its  real  effect  is  merely 
to  increase  the  current  in  the  line  by  a  small  amount,  usually 
less  than  15  per  cent.,  and  to  demand  increase  of  excitation 
at  the  dynamo.  If  this  is  so  designed  as  to  regulate  badly, 
an  inductive  load  will  render  it  difficult  or  impossible  to  keep 
a  uniform  voltage.  On  the  other  hand,  a  generator  capable 
of  holding  its  voltage  from  no  load  to  a  full  and  inductive 
load  with  an  increase  of  only  15  or  20  per  cent,  in  the  exciting 
current  will  usually  give  no  trouble  whatever  with  reasonable 
attention  to  the  regulators. 

The  total  net  result  of  inductance  in  line  and  load  is  to 
call  for  a  well-designed  generator  with  good  inherent  regula- 


460  ELECTRIC  TRANSMISSION  OF  POWER. 

tion  and  a  reasonable  margin  of  capacity.  One  who  know- 
ingly installs  anything  else  deserves  all  the  troubles  that 
inductance  can  produce. 

Rise  in  voltage,  on  throwing  off  the  load  or  through  distor- 
tion of  the  current  wave  by  an  inductive  load,  can  be  reduced 
to  insignificance  by  employing  a  proper  generator  as  just 
noted.  Aside  from  this,  a  mixed  load,  particularly  if  it  con- 
sists in  part  of  synchronous  motors,  seldom  has  a  bad*  power 
factor  or  great  and  sudden  changes  in  its  amount.  Exception 
must  here  be  made  with  respect  to  the  constant  current  trans- 
former systems  exploited  of  late  in  connection  with  series 
alternating  arc  lamps.  These  unless  fully  loaded  give  a 
severely  inductive  load,  and  must  be  thrown  upon  the  circuit 
very  carefully  to  avoid  serious  fluctuations  of  voltage. 

As  regards  static  disturbances,  few  overhead  systems  have 
capacity  enough  to  give  cause  for  alarm.  Difficulties  are  to 
be  looked  for  chiefly  on  very  long  lines,  longer  than  any 
now  in  use,  and  those  composed  in  part  of  underground  or 
submarine  cables.  In  these  cases  one  may  sometimes  know 
the  conditions  well  enough  to  calculate  the  actual  result  in 
rise  of  voltage.  More  often  the  data  are  incomplete,  and  the 
simplest  way  out  of  the  difficulty  is  to  try  the  effect  of  vary- 
ing the  capacity  of  the  system  before  it  goes  into  regular 
operation.  If  the  addition  of  a  condenser,  say  of  one-third 
microfarad,  makes  a  sharp  variation  in  the  voltage,  look  out  for 
resonance  and  investigate  the  capacity  of  the  system,  step  by 
step.  A  change  of  capacity  or  inductance  can  be  made  suf- 
ficient to  avert  any  serious  danger  of  resonance  under  ordinary 
conditions.  Resonance  chargeable  to  the  variation  of  har- 
monics under  changes  of  load  and  to  changes  in  inductance 
and  capacity  due  to  apparatus  used  on  the  system  is  hard  to 
foresee  and  must  be  treated  symptomatically  when  it  chances 
to  appear. 

We  may  now  pass  to  the  practical  calculation  of  a  line. 

CASE  I. — The  simplest  possible  case  is  the  transmission  of 
direct  current  over  a  short  distance.  Let  it  be  required  to 
deliver  200  HP  to  a  machine  shop  for  operating  machinery  and 
a  small  tramway  over  a  distance  of  one  mile  and  a  half,  power 
to  be  about  equally  divided  between  small  motors  and  tram- 
way. Source  of  power — water  power — owned  outright  by  the 


THE  LINE.  461 

company,  and  ample  in  amount.  Load  on  small  motors 
steady,  on  tramway  very  variable.  In  a  case  of  this  kind  the 
use  of  direct  current  is  advantageous,  since  with  plenty  of 
power  considerable  drop  can  be  permitted,  and  the  use  of  any 
alternating  system  would  involve  changing  to  direct  current 
for  the  tramway.  For  use  in  a  machine  shop  the  speed  of  the 
motors  need  not  be  very  uniform,  so  that  there  will  be  no 
especial  difficulties  of  regulation.  On  account  of  size  of 
motors  500  to  600  volts  will  be  the  highest  permissible  voltage. 

There  are  several  courses  open  in  determining  the  drop.  The 
easiest  is  to  assume  for  the  final  voltage  such  a  value  as  will  give 
the  greatest  drop  compatible  with  convenient  regulation,  and 
for  the  initial  voltage  that  delivered  by  a  standard  railway  gener- 
ator. A  special  generator  is  objectionable  on  account  of  both 
first  cost  and  difficulty  of  repairs.  The  generator  can  be  counted 
ua  for  600  volts  maximum.  It,  therefore,  we  run  it  at  a  normal 
no-load  voltage  of  550,  and  over  compound  about  10  per  cent., 
we  shall  get  no  more  than  about  550  volts  at  the  motors. 
During  the  time  that  the  tramway  is  working  we  can  let  the 
voltage  fall  to  500,  for  which  the  tramway  motors  give  their 
normal  output,  without  interfering  materially  with  the  regula- 
tion of  the  other  motors.  We  take  the  maximum  drop,  then,  at 
100  volts.  The  amount  of  power  to  be  delivered  at  full  load 
is  200  HP. 

Taking  the  average  commercial  efficiency  of  the  motors  at 
80  per  cent.,  the  electrical  HP  to  be  delivered  is  250,  which 
equals  very  nearly  187,500  watts  at  500  volts.  The  current, 
therefore,  is  375  amperes.  To  allow  for  sag,  joints  and  acci- 
dental variations,  we  should  take  the  mile  as  about  5,400  ft. 
instead  of  5,280.  We  are  now  ready  to  determine  the  cross 
section  of  the  wire  by  the  formula  already  given.  We  then 
have 

cm  =     16,200  X  ii  X  375    =  668 
100 

Looking  now  in  our  wire  table,  we  find  that  the  nearest  con- 
venient approach  to  this  cross  section  is  three  oooo  wires,  each 
of  211,600  cm.  We  might  reinforce  these  by  a  No.  5  wire 
and  get  almost  exactly  the  amount  required,  but  the  game  is 
hardly  worth  the  candle,  since  we  are  working  for  minimum 


462  ELECTRIC    TRANSMISSION   OF  POWER. 

first  cost,  and  a  very  trifling  increase  of  drop  is  not  worth 
considering,  particularly  as  the  maximum  load  will  be  for  only 
a  few  minutes  at  a  time.  A  nominal  200  KW  generator  will 
serve  our  purpose  if  built,  as  standard  railway  generators  are, 
to  stand  small  and  temporary  overloads  at  any  time.  The 
cost  of  the  copper  for  such  a  circuit  is  considerable;  48,600 
ft.  of  No.  oooo  will  be  required,  weighing,  when  insulated,  as 
it  generally  wou'd  be  in  running  into  a  town,  about  750  Ibs. 
per  1000  ft.  The  total  weight  would  then  be  36,450  Ibs., 
costing,  at  15  cts.  per  lb.,  $5,467,  quite  nearly  $22  per  HP 
delivered.  Occasionally  it  would  be  advisable  to  allow 
greater  drop  than  that  here  assumed. 

CASE  II.  A  cotton  mill  is  operated  by  steam,  coal  costing 
about  $5  per  ton,  the  actual  brake  horse-power  required  t) 
drive  the  machinery  being  660.  Speed  must  be  constant, 
load  is  very  uniform,  and  any  reduction  of  available  horse- 
power cuts  down  the  output,  all  of  which  is  badly  needed  to 
fill  contracts.  A  cheap  water  power,  distant  22,000  ft.,  is 
acquired  by  the  mill  owners.  Sixteen  thousand  cubic  ft.  of 
water  per  minute  is  available,  and  the  greatest  head  that  can  be 
utilized  is  30  ft.  At  the  existing  price  for  coal  electrical  trans- 
mission will  certainly  pay.  The  full  660  HP  must  be  deliv- 
ered, or  the  power  will  have  to  be  supplemented  by  steam, 
unless  part  of  the^  looms  are  shut  down. 

The  given  amount  of  water  will  produce,  when  utilized  in  first- 
class  turbines,  almost  exactly  800  mechanical  HP.  Two  tur- 
bines are  used  for  security  against  breakdown,  and  each  runs  150 
revolutions  per  minute.  The  commercial  efficiency  of  the  plant 

must  then  be  at  least  =  82.5.  per  cent.     Taking  300  KW 

800 

units  for  generators  and  motors  we  can  be  sure  of  a  commer- 
cial efficiency  of  .93  in  each  machine.  This  limits  the  effi- 
ciency of  the  line  to  not  less  than  95.4.  It  is  evident  at  once 
that  direct  coupled  generators  must  be  used,  since  the  2  or  3 
per  cent,  lost  in  belting  would  be  quite  inadmissible. 

As  to  voltage,  it  is  evident  that  the  highest  available  in  a 
300  KW  unit  should  be  used.  Without  going  into  special 
windings,  the  voltage  would  be,  say,  4,000  over  compounded 
about  5  per  cent,  giving  4,200  at  the  brushes.  At  the  as- 
sumed efficiency  of  the  motors  there  must  be  delivered  to  them 


THE  LINE.  463 

529  KW.  At  4.6  per  cent,  drop  in  the  line  132  amperes  must 
be  the  current.  All  question  of  inductive  drop  may  be  neg- 
lected, since  the  motor  field  can  be  adjusted  to  compensate 
for  it.  Taking  the  actual  length  of  circuit  at  45,000  ft,  and 
employing  the  formula  as  in  Case  I.,  the  cross  section  of  con- 
ductor comes  out  as  355,100  cm. 

This  is  very  nearly  met  by  three  No.  oo  B.  G.  W.  wires. 
Wire  drawn  to  this  British  v:ire  gauge  can  be  readily  obtained 
without  increased  cost.  The  combined  section  of  the  three 
is  but  a  little  over  2  per  cent,  more  than  that  required,  hence 
wilf  serve  excellently.  Supposing  bare  wire  to  be  used, 
the  weight  of  each  conductor  per  thousand  feet  is  369  Ibs. 
The  total  weight  of  copper  is  then  49,815  Ibs.,  costing,  at  15 
cents  per  lb.,  $7,472.  Any  increase  in  the  loss  in  the  line  will 
cause  a  failure  to  deliver  the  required  power  and  necessitate 
the  operation  of  a  subsidiary  steam  plant,  or  the  shutting  down 
of  nearly  a  score  of  looms  for  each  per  cent,  decrease  in 
efficiency.  Doubling  the  line  loss  would  save  $3,736,  but  at 
a  cost  and  trouble  of  a  40  HP  steam  plant,  or  the  loss  of  the 
output  of  75  or  80  looms.  In  a  case  of  this  kind  the  easiest 
way  of  reducing  the  cost  of  line  would  be  to  use  the  three- 
phase  synchronous  system,  which  would  save  a  quarter  of  the 
above  copper  at  the  same  efficiency.  Generators  and  motors 
of  more  than  4,000  volts  would  give  even  better  results  if 
thoroughly  reliable,  as  they  now  would  be  for  units  of  the 
size  in  question.  Using  10,000  volt  three-phase  generators 
and  motors,  of  course  of  the  stationary  armature  type,  one 
could  keep  the  total  loss  in  the  line  down  to  2.5  per  cent,  at  a 
cost  for  copper  of  a  little  less  than  $2,000,  at  the  price  assumed 
for  copper  in  this  example. 

CASE  III.  It  is  desired  to  deliver  for  general  service  in  a 
city  1,000  KW  from  an  ample  water  power  20  miles  distant. 
Power  is  to  be  used  for  arc  and  incandescent  lighting,  and  a 
motor  service  of  unknown  amount,  possibly  as  great  as  500 
HP;  3,000  to  4,000  HP  can  be  obtained  at  the  power  station; 
and  allowance  must  be  made  for  extensions. 

On  account  of  reduced  cost  of  line  the  three-phase  system 
is  desirable,  effecting  the  distribution  by  secondary  mains  at 
about  220  volts  between  wires,  using  a  balance  wire  for  lamps. 
This  requires  a  little  less  than  30  per  cent,  of  the  copper 


464  ELECTRIC   TRANSMISSION   OF  POWER. 

needed  on  the  two-wire  system  with  the  same  voltage  at  lamps. 
Evidently  raising  and  reducing  transformers  should  be  usedr 
and  we  may  allow,  to  keep  within  quite  conservative  bounds, 
10,000  volts  on  the  line.  We  should  first  decide  on  the  gen- 
erators. Four  hundred  KW  is  a  convenient  size,  three  being 
used  for  the  total  plant.  The  voltage  should  be  moderate; 
say,  500  volts.  With  this  preliminary,  let  us  figure  the  line. 
We  can  conveniently  lose  about  10  per  cent,  of  the  energy, 
say,  5  per  cent,  in  the  two  banks  of  transformers  and  5  per 
cent,  loss  in  the  line.  The  voltage  at  the  receiving  end  will 
be  9,500,  with  10,000  at  the  raising  transformers.  As  to  reg- 
ulation, the  distributing  system  is  worked  from  a  sub-station 
where  the  regulating  is  done. 

Since  the  copper  required  is  three-fourths  of  that  needed  for 
a  two-wire  system  at  the  same  voltage,  each  of  the  three  wires 
will  have  one-half  the  cross  section  of  either  wire  on  the  two- 
wire  system.  We  may,  therefore,  use  our  previous  formula  if 
we  take  account  of  this  factor  one-half.  The  simplest  way  of 
doing  it  is  to  take  the  single  distance  of  transmission  instead 
of  the  double  distance.  As  to  the  current  in  the  formula,  it 
must  be  the  equivalent  single-phase  current.  If  we  write 


instead  of  C,  we  shall  avoid  all  confusion  on  this  score.  Our 
formula  then  becomes 

OL 

£>  x  ii  x  v 

—y-         -, 

wherein  D  is  the  distance  of  transmission,  W  the  watts  deliv- 
ered or  received,  V  the  voltage  of  delivery  or  reception,  and 
V^  the  volts  lost  in  the  line.  In  our  case  D  —  108,000  (the 
mile  being  taken  as  5,400  ft.),  V  =  9,500,  V^  =  500,  W  = 
1,000,000.  Then  c.  m.  =  250,000  (nearly). 

But  we  must  remember  that  our  load  is  a  mixed  one  of 
lights,  and  probably  both  synchronous  and  induction  motors. 
Hence  it  will  have  an  inductance.  Hence  a  current  somewhat 
larger  than  that  given  by 

W 
V 


THE   LINE.  465 

must  be  taken.  With  fairly  good  luck  in  our  proportion  of  lights 
and  synchronous  motors,  5  per  cent,  increase  will  be  enough  to 
allow.  At  worst  it  could  hardly  be  greater  than  10  per  cent., 
which  would  mean  a  very  small  increase  in  ohmic  drop.  Tak- 
ing it  at  5  per  cent.,  we  add  that  amount  to  our  cross  section 
and  obtain  262,500  c.  m.  The  nearest  approach  to  this  with 
standard  wires  is  two  No.  oo  =  266,000  c.  m. 

Now  as  to  inductive  drop.  At  worst  the  impedance  factor 
of  the  line  will  not  be  greater  than  1.79,  with  60  periods  and 
wires  strung  18"  apart.  Therefore,  a  rise  of  less  than  4  per 
•cent,  in  our  generator  voltage  will  take  care  of  the  inductance 
in  the  line.  As  to  capacity,  each  pair  of  wires  will  have  a 
•capacity  of  less  than  y-J-^  microfarad  per  mile,  so  that  we  need 
not  borrow  trouble  on  the  score  of  resonance.  The  effect  of 
total  inductance  on  the  generator  is  small,  since  the  power 
factor  of  the  system  is  likely  to  be  ns  ureat  as  .95,  and  if  the 
generator  is  even  tolerably  well  designed,  a  few  per  cent. 
increase  in  excitation  will  compensate  for  this.  Now  comput- 
ing the  total  amount  of  copper  we  find  it  to  be  261,144  Ibs., 
costing,  at  15  cents  per  Ib.  for  bare  wire,  $39,171.  Except 
under  unfavorable  climatic  conditions  one  would  be  likely  to 
goto  15,000  or  20,000  volts  if  raising  transformers  were  to  be 
used  at  all.  These  would  require  respectively  44.4  per  cent. 
and  25  per  cent,  of  the  total  weight  of  copper  indicated  for 
10,000  volts.  The  computation  of  the  line  would  be  precisely 
similar  to  that  just  explained. 

A  very  simple  and  accurate  formula  for  giving  the  total 
weightof  a  three-phase  circuit  directly,  neglecting  inductance,  is 


W 

TOO  zy-F 


Dm  being  the  distance  of  transmission  reckoned  in  thousands 
of  feet. 

We  now  have  a  working  line  laid  out,  and  feel  sure  that  no 
contingencies  we  are  likely  to  meet  will  cause  the  drop  to 
vary  much  from  5  per  cent.,  since  within  the  probable  range  of 
power  factor  that  can  be  found  in  a  mixed  plant  both  line  and 
generators  are  well  able  to  take  care  of  the  variations.  The 
net  result  of  inductance  has  been  to  increase  our  allowance  of 


466  ELECTRIC    TRANSMISSION   OF  POWER. 

copper  by  5  per  cent.,  and  to  require  10  or  15  per  cent,  margin 
of  excitation  and  current  capacity  in  our  generator. 

This  may  seem  a  rough  and  ready  process  for  figuring  a 
transmission  line,  but  it  gives  a  safe  and  conservative  result 
as  nearly  exact  as  the  data  generally  available  for  a  new  plant 
permit.  If  the  capacities  and  inductances  of  the  future  sys- 
tem could  be  predetermined,  we  could  take  their  vector  sum 
and  compute  a  small  correction  to  our  factor  of  safety  in  the 
generator.  If  the  future  load  were  exactly  known  we  might 
wish  to  change  the  drop  slightly.  No  exact  data  are  available 
under  ordinary  circumstances,  hence  we  must  rely  on  our 
judgment  in  selecting  the  drop  and  factor  of  safety  in  the 
generator,  which,  unless  it  be  downright  bad  in  design,  will 
have  margin  enough  to  meet  any  reasonable  requirements. 

A  convenient  formula  giving  cost  directly  is 

W 


wherein  P  is  the  total  cost  of  the  conductors  in  dollars  and  / 
the  current  price  of  bare  wire  in  cents  per  Ib.  Since  an 
aluminium  line  has  almost  exactly  half  the  weight  of  a  copper 
line  of  equal  conductivity,  this  formula  and  the  preceding  one 
may  be  used  for  computing  an  aluminium  line  by  simply  writ- 
ing the  factor  2  in  the  denominator. 


CHAPTER    XIII. 

LINE    CONSTRUCTION. 

THE  first  consideration  is  the  general  question  of  location. 
Other  things  being  equal  it  is  obvious  that  a  direct  line  is  the 
best,  but  as  a  matter  of  fact  it  is  seldom  altogether  practicable. 
A  line  must  above  all  things  be  secure  against  interruptions, 
and  with  this  in  view  both  the  location  and  the  constructional 
features  should  be  determined. 

In  smooth  and  easy  country  a  nearly  straight  line  can  usu- 
ally be  laid  out.  For  large  plants  carrying  large  amounts  of 
power  at  high  voltages  it  is  often  desirable  to  buy  the  right  of 
way  outright.  Such  has  mainly  been  the  policy  pursued  in  the 
transmission  from  Niagara  to  Buffalo,  and,  while  expensive,  it 
gives  an  absolute  command  of  the  situation.  In  some  States 
electric  light  and  power  companies  are  given  the  right  of 
eminent  domain  to  make  such  ownership  possible. 

In  cases  wherein  the  purchase  of  such  a  location  is  imprac- 
ticable or  would,  as  often  happens,  involve  very  serious  expense 
the  best  thing  is  to  secure  right  of  way  along  the  public  roads, 
so  far  as  they  can  be  conveniently  utilized,  and  right  of  way 
for  the  pole  line  through  such  private  property  as  may  be  in 
the  contemplated  route.  Rights  along  the  public  roads  are 
very  desirable,  as  giving  capital  facilities  for  line  inspection 
and  repair  without  expense.  Right  of  way  merely  for  the 
line  across  private  lands,  with  proper  facilities  for  access,  can 
generally  be  cheaply  secured.  Many  owners  are  public-spirited 
enough  to  give  it  for  the  asking,  or  for  very  reasonable  com- 
pensation when  a  strip  of  land  has  to  be  taken  for  a  roadway. 

In  small  transmissions  the  public  roads  are  most  desirable 
as  a  route,  using  private  lands  only  for  occasional  short  cuts. 

Since  a  good  road  along,  the  pole  line  is  highly  desirable, 
the  route  should  be  taken  through  clear  and  accessible  country, 
so  far  as  is  possible. 

Places  to  be  avoided  when  possible,  even  by  a  detour,  are 


468  ELECTRIC    TRANSMISSION  OF  POWER. 

marshes,  where  poles  are  always  hard  to  set  and  maintain  and 
roads  are  difficult  to  construct;  heavily  wooded  country, 
where  there  is  constant  danger  to  the  line  from  falling 
branches  and  the  like;  and  rough  rocky  slopes,  where  construc- 
tion is  difficult  and  the  line,  when  constructed,  is  highly  inac- 
cessible. Sometimes  the  topographical  conditions  are  such 
that  these  difficulties  have  to  be  met,  but  they  are  always 
serious. 

In  a  wooded  region  the  only  proper  plan  is  to  secure  right 
of  way  broad  enough  to  permit  clearing  away  the  trees  so 
that  they  cannot  interfere  with  the  line  wires,  even  were 
branches  to  be  blown  off  in  a  storm.  Nothing  short  of  a 
hurricane  sufficient  to  blow  down  large  trees  should  possibly 
be  able  to  cause  trouble,  and  when  the  neighboring  trees  are 
dangerously  high  careful  watch  should  be  kept  and  any  weak 
or  decaying  tree  at  once  cut  down.  The  right  of  way  may  be 
somewhat  expensive,  but  the  service  must  not  be  liable  to 
interruption  by  so  probable  a  thing  as  the  breaking  of  a 
branch.  It  must  be  remembered  that  in  high  voltage  trans- 
missions a  twig  as  big  as  a  lead-pencil  may  by  falling  across  the 
line  start  an  arc  that  will  shut  down  the  plant.  Sometimes 
the  use  of  extra  long  poles  may  enable  one  to  carry  the  wires 
clear  of  possible  obstructions  of  this  sort. 

In  mountainous  regions  poles  may  have  to  be  set  in  very 
bad  locations,  and  sometimes  for  long  stretches  every  hole 
may  have  to  be  blasted  at  a  cost  of  $5  to  $10  per  hole,  but 
such  contingencies  are  not  very  common  and  may  often  be 
avoided  by  a  moderate  detour.  It  is  better  to  go  around  a 
mountain  than  over  it,  unless  the  distance  is  considerably 
greater.  When  these  questions  arise  they  should  be  answered 
by  preliminary  estimates.  The  country  should  be  carefully 
inspected  and  the  relative  costs  of  various  routes  looked  into. 
For  a  uniform  country  the  cost  of  poles  and  construction  is 
directly  as  the  distance,  and  the  cost  of  copper  directly  as  the 
square  of  the  distance. 

In  case  the  direct  line  leads  into  difficult  country — over,  for 
example,  a  rocky  hill  where  the  poles  would 'be  hard  to  place 
and  much  blasting  would  have  to  be  done — a  detour  often  may 
cheapen  construction.  A  brief  computation  will  give  the 
facts.  Suppose  a  lo-mile  transmission  of  about  500  KW  at 


LINE   CONSTRUCTION.  469 

10,000  volts,  for  simplicity  assumed  to  be  on  the  monophase 
system.  The  line  would  have  to  be  about  No.  o  wire  for  say  6 
per  cent,  loss,  and  the  total  weight  of  copper  would  be  about 
33,000  Ibs.  Suppose  the  average  cost  of  poles  and  insulators 
in  position  to  be  $5  in  the  open  country,  but  that  the  direct 
route  lies  for  a  mile  over  a  rough  hill,  where  holes  would  have 
to  be  blasted  and  poles  would  be  difficult  to  place.  The  extra 
cost  of  this  mile  might  readily  be  $500  to  $600.  Now  if  a 
deviation  of  a  mile  would  clear  this  hill,  it  would  probably 
pay  to  abandon  the  direct  route.  By  taking  the  shortest 
available  course  the  actual  increase  in  the  length  of  the  route 
would  probably  not  exceed  half  a  mile.  This  would  increase 
the  weight  of  copper  for  the  same  loss  by  about  10  per  cent., 
$495  at  I5C-  Per  lb.f  anc*  would  increase  the  cost  of  the  pole 
line  by  about  $250  more.  In  such  a  case  the  increased 
accessibility  of  the  line,  and  the  lessened  cost  of  providing  a 
road  for  inspection  and  repairs,  would  more  than  compensate 
for  the  small  difference  in  expense. 

The  same  reasoning  holds  with  respect  to  avoiding  other 
obstacles  by  making  detours.  It  often  pays  to  go  somewhat 
out  of  the  way  to  utilize  the  public  roads,  to  cross  rivers  on 
existing  bridges,  and  so  forth.  A  few  experiments  on  the 
route  constructed  on  paper,  after  careful  inspection  of  the 
country,  will  usually  show  the  most  advantageous  line  to 
follow.  The  old  and  simple  process  of  sticking  pins  in  the 
map  and  following  up  the  line  with  thread  is  generally  the 
easiest  way  of  getting  the  approximate  distances. 

In  mountainous  country  a  direct  line  is  often  out  of  the 
question,  and  the  line  has  to  conform  to  existing  trails  with 
such  short  cuts  as  may  be  possible.  An  occasional  long  span 
will  sometimes  lessen  the  cost  of  the  line  materially.  Rivers 
and  lakes  often  form  very  serious  obstacles  to  line  construc- 
tion and  call  for  much  skillful  engineering.  The  former  can 
often  be  crossed  on  existing  bridges  or  by  long  spans,  which 
will  be  discussed  later,  but  the  latter  usually  have  to  be  gone 
around,  although  sometimes  cables  may  have  to  be  carried 
under  water.  If  the  lake  is  close  to  either  end  of  the  line,  so 
that  it  can  be  crossed  by  a  cable  at  moderate  voltage,  it  is 
sometimes  advisable  to  do  so.  In  two  important  cases  that 
have  come  to  the  author's  notice  this  was  the  only  practicable 
escaoe  from  the  watery  barrier. 


470 


ELECTRIC   TRANSMISSION  OF  POWER. 


Nearly  all  long  lines  have  to  encounter  more  or  less  serious 
obstacles  of  the  sorts  mentioned,  and  as  a  rule  they  cause 
considerable  deflections  from  a  straight  course.  Sometimes 
deviations  are  desirable  merely  as  the  cheapest  way  of  reach- 
ing en  route  localities  where  power  is  to  be  distributed. 


LINE    WIRE. 

As  already  mentioned,  copper  is  the  best  and  most  usual 
material  for  conductors;  soft-drawn  copper  under  ordinary 
circumstances,  hard-drawn  when  extra  strength  is  desirable. 
No  other  material  gives  so  advantageous  a  combination  of 
conductivity  and  tensile  strength  for  nearly  all  purposes. 
The  tensile  strength  of  the  copper  is  raised  by  hard  drawing 
from  about  34,000  to  35,000  Ibs.  per  square  inch  to  60,000  or 
even  70,000,  and  the  resistance  is  only  raised  2  to  4  per  cent., 
the  latter  amount  only  in  small  sizes.  Sometimes  a  medium 
hard-drawn  wire  is  used  having  a  tensile  strength  of  say  45,000 
to  50,000  Ibs.  per  square  inch.  Such  wire  is  materially 
stronger  than  the  annealed  wire  and  yet  is  much  easier  to 
handle  than  such  hard-drawn  wire  as  is  used  for  trolley  wires. 

For  line  copper  the  wire  should  be  free  from  scale,  flaws, 
seams,  and  other  mechanical  imperfections.  It  should  be  very 
close  to  its  nominal  gauge,  variations  of  i  to  2  mils  being  the 
largest  which  should  be  tolerated,  and  should  be  within  2  per 


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LINE   CONSTRUCTION.  47 * 

cent,  or  less  of  standard  conductivity,  as  given  for  pure 
copper  in  tables  of  wire. 

The  foregoing  table  gives  the  standard  electrical  and  me- 
chanical constants  of  the  sizes  of  wire  used  in  power  trans- 
mission work. 

The  various  constants  should  none  of  them  fall  short  of  these 
tabulated  values  by  more  than  2  per  cent.  Specially,  the 
resistances  should  not  be  in  excess  of  those  given  by  more 
than  this  amount. 

For  hard-drawn  copper  wire  the  resistance  never  ought  to 
exceed  the  tabular  values  by  more  than  5  per  cent,  and  the 
tensile  strength  should  not  fall  short  of  1.75  times  the  values 
given  for  annealed  wire. 

When  in  use,  wire  is  subject  to  serious  mechanical  strains,, 
due  in  the  first  place  to  its  weight  and  normal  tension,  second 
to  variations  in  tension  by  change  of  temperature,  and  third 
to  extraneous  loads  like  ice  and  wind  pressure,  separately 
or  combined.  These  last-mentioned  strains  are  sometimes 
formidable  and  must  be  carefully  taken  into  account,  partic- 
ularly in  cold  climates. 

When  a  wire  is  suspended  freely  between  supports  it  takes 
a  curve  known  technically  as  the  catenary.  The  exact  solution 
of  its  properties  is  very  difficult,  but  for  the  case  in  hand  the 
catenary  comes  very  close  to  the  parabola,  a  much  simpler 
curve  to  compute ;  and  based  on  this  approximation  the  follow- 
ing simple  deductions  can  be  made:  If  a  wire  be  stretched 


FIG.  221. 

between  the  points  A  and  B,  Fig.  221,  it  assumes  the  curve 
A  D  B.  The  thing  to  be  determined  is  the  relation  between 
the  length  A  B  (which  we  may  call  Z,  the  length  of  span), 
the  vertical  deflection  d,  at  the  middle  point  of  the  span, 
and  the  tension  on  the  wire  at  A  or  B  as  a  function  of  its 
weight.  This  relation  is  as  follows: 

j,  _  D  w  .  , 

or  transposing, 

=  S~T (2) 


472  ELECTRIC   TRANSMISSION  OF  POWER. 

Here  L  is  the  length  of  the  span  in  feet,  d  the  central 
deflection  in  feet,  w  the  weight  of  the  wire  in  pounds  per  foot, 
and  T  the  maximum  tension  on  the  wire  in  pounds. 

These  equations  show  that  with  a  given  wire  the  tension 
varies  inversely  as  the  deflection  for  a  given  span,  and  that  for 
a  given  tension  and  wire,  the  deflection  must  increase  with 
the  square  of  the  span.  Obviously,  shortening  the  span  and 
increasing  the  deflection  eases  the  strain  on  the  wire  and 
renders  the  construction  more  secure,  but  shortening  the  span 
adds  considerably  to  the  cost,  and  increasing  the  deflection 
increases  the  danger  of  the  wires  swinging  in  the  wind  and 
touching  each  other.  To  prevent  this  the  deflection  should 
not  much  exceed  twice  the  horizontal  distance  between  wires. 

The  application  of  the  formulae  can  best  be  shown  by  an 
example.  Suppose  we  are  stringing  No.  oo  wire  on  poles  100 
feet  apart  What  is  the  least  deflection  allowable  with  a 
factor  of  safety  of  4?  This  means  that  T  must  not  exceed 
one-fourth  the  breaking  strain  of  the  wire,  which  fraction 
from  the  table  is  888  Ibs.  The  weight  per  foot  from  the  table 
is  .4  Ib.  Substituting  in  equation  (2)  we  have: 


This  minimum  deflection  should  not  be  exceeded  in  this  case, 
and  hence  must  be  applicable  to  the  lowest  temperature  to 
which  the  line  is  to  be  exposed.  At  whatever  temperature 
the  wire  is  strung,  enough  deflection  should  be  allowed  so  that, 
as  the  wire  contracts  in  cold  weather,  the  above  minimum 
should  not  be  passed. 

The  total  length  of  wire  in  the  catenary  is  approximately 


or  transposing  for  the  value  of  d, 


Wherein  D  is  the  actual  length  of  wire,  and  L  the  span. 

From  these  formulae  we  can  figure  </for  any  temperature. 

The  coefficient  of  expansion  of  copper  is  .0000095  of  its 
length  per  degree  Fahrenheit,  so  that  we  can  get  at  once  the 
length  for  any  temperature. 


LINE   CONSTRUCTION.  473 

If  the  wire  we  are  considering  is  strung  at  75°  F.  and  is  to 
encounter  a  minimum  temperature  of  —5°  F.,  enough  deflec- 
tion must  be  allowed  at  the  former  temperature  to  bring  the 
deflection  at  —5°  F.  to  the  value  just  obtained.  The  length 
of  wire  at  the  lower  temperature  is  from  (3), 

Z1  ;=  100  +  8  I57)'  =  100.0096. 
300 

At  75°  F.  this  length  would  be  increased  by  100.0086  x 
.0000095  x  80  feet,  and  hence  the  new  value  of  Z1  would  be 
100.076  feet.  The  deflection  corresponding  to  this  is  found 
from  (4)  as  follows : 

d  =  A/3°°  *  -°76  =  1.69  ft.  =  20.28  inches. 

A  large  allowance  in  deflection  must  therefore  be  made  for 
such  variations  in  temperature  as  are  likely-ta  be  encountered 
in  northern  climates. 

The  changes  in  deflection  due  to  changes  of  temperature  are 
found  in  practice  to  be  somewhat  lessened  by  the  fact  that  the 
wire  as  strung  is  under  tension  due  to  its  weight,  which  modi- 
fies its  expansion  and  contraction. 

This  matter  of  temperature  is  unfortunately  not  all  that 
must  be  looked  out  for.  We  have  fully  taken  care  of  the 
weight  of  the  wire  itself,  but  it  is  exposed  to  other  and  some- 
times dangerous  forces  in  the  weight  of  the  ice  coating  that 
is  to  be  feared  in  winter  and  the  strain  of  wind  pressure  on 
the  wire  either  bare  or  ice-coated. 

Taking  up  these  in  order,  let  us  suppose  the  wire  to  become 
coated  with  ice  to  the  thickness  of  half  an  inch,  quite  a 
possible  contingency  in  severe  winter  storms.  A  layer  of  ice 
of  this  thickness  would  weigh  0.54  Ib.  per  linear  foot,  thus 
loading  the  wire  with  more  than  its  own  weight.  Assuming 
this  load  at  the  minimum  temperature  of  — 5°  for  which  the 
assumed  deflection  was  0.57  foot,  the  tension  of  the  ice-loaded 
wire  becomes  from  (i), 

r=(iop)'X.94  =  2,05libs. 

»X  .57 

This  is  dangerously  large,  far  beyond  the  elastic  limit  of  the 
wire,  and  more  than  likely  to  bring  down  weak  joints. 


474  ELECTRIC   TRANSMISSION  OF  POWER. 

And  beyond  this  the  wind  pressure  must  be  considered. 
This  may  be  taken  as  acting  at  right  angles  to  the  weight  of 
the  wire  and  adding  materially  to  the  resulting  total  stress. 
The  total  pressure  P  on  a  wire  is,  per  foot,  approximately 
P  =  .05  /  Z>,  where/  is  the  normal  pressure  of  the  wind  per 
square  foot  and  D  is  the  diameter,  of  the  wire  in  inches.  / 
varies  from  a  few  ounces  per  square  foot  in  light  breezes  to  40 
or  50  pounds  in  a  hurricane. 

Assuming  40  Ibs.  as  the  greatest  pressure  likely  to  be 
encountered,  we  can  at  once  find  its  effect  on  the  line  under 
consideration.  For  our  No.  oo  wire 

P  —  .05  X  40  X  .364  =  .728  Ibs. 

This  pressure  is  combined  with  the  weight  of  the  wire  as  a 
force  acting  at  right  angles,  hence  the  resultant  stress,  which 
we  may  call  W,  is 

W  =  *JW*  +  p*  =   V(.4)2+  (.728)"  =  -83. 

This,  from  the  example  given,  is  obviously  a  dangerous  strain 
on  the  wire.  But  the  combination  of  even  half  the  normal 
wind  pressure  just  assumed  with  an  ice-coated  wire  would  be 
disastrous.  Taking  the  ice  as  half  an  inch  thick  as  before, 

D  —  1.36,  P  =  .05  X  20  X  1.36  =  1.36, 
and 

W  =  V(.94)'  +  (i-36)f  =  *'*S. 
Substituting  in  (i) 


This  is  over  the  breaking  weight  of  the  wire,  which  must  con- 
sequently give  way,  and  would  almost  infallibly  wreck  the  line 
in  so  doing.  This  means  that  the  factor  of  safety  of  4,  assumed 
at  the  start,  is  too  small  for  due  security.  It  is  sufficient  for  a 
moderate  climate,  where  high  winds  are  rare,  but  5  is  generally 
preferable,  while  7  or  8  should  be  used  in  cold  and  exposed 
regions.  It  must  be  remembered  that  joints  are  weak  points 
in  the  wire;  a  carefully  soldered  Western  Union  joint  has 
only  about  85  per  cent,  the  strength  of  the  wire. 

The  same  process  that  served  to  take  account  of  an  ice  coat- 


LINE   CONSTRUCTION.  475 

ing,  /*.  e. ,  adding  the  distributed  load  to  the  weight  of  the  wire, 
can  be  readily  applied  to  finding  conditions,  of  safety  in  the 
use  of  bearer  wires  carrying  the  conductor  suspended  from 
them. 

An  interesting  corollary  to  these  computations  is  finding  the 
maximum  length  of  span  which  can  safely  be  used  in  an 
emergency  such  as  crossing  a  river  or  canon.  Suppose  we 
use  simply  hard-drawn  copper  wire  of  the  same  size  as  before. 
Its  ultimate  tenacity  is  about  6,270  Ibs.  Using  it  with  a  factor 
of  safety  of  6,  the  permissible  value  of  T  becomes  1,045  Ibs. 
W\s  as  before  0.4  Ib.  and  we  will  assume  that  for  the  purpose 
in  hand  the  wires  are  spread  and  the  deflection  is  permitted  to 
be  io  feet.  From  (i)  we  have  for  the  permissible  length  of  span 

/  Q       rri     J 

L=  j/iZr.    (5) 

/y     IY 

Substituting  the  above  values  of  the  known  quantities,  we  have 


T  —    A  /8  X  I045  X  io  _     e      ,    t    t 
J^>  —   4/  Zr —  457  "T  icet. 

I/  A 

f  '  T"  * 

10,  however,  is  a  preferable  factor  of  safety,  which  corresponds 
to  a  length  of  span  of  354  feet.  In  extreme  cases  a  bearer  of 
steel  cable  may  be  used,  of  the  highest  available  tenacity,  and 
carrying  the  copper  line  wire  to  secure  the  requisite  conduc- 
tivity, or  a  steel  or  silicon  bronze  wire  may  be  used  alone : 
the  conductivity  being  made  up  elsewhere  in  the  line  to  the 
desired  general  average.  The  steel  is  rather  the  more  reliable 
of  the  two,  but  is  more  likely  to  deteriorate  through  rusting. 
An  ultimate  tenacity  of  150,000  Ibs.  per  sq.  in.  is  the  limit  for 
either  material,  with  factor  of  safety  of  io  for  practical 
working. 

Now  assuming  No.  oo  silicon  bronze  or  its  equivalent  in 
steel  cable  and  the  same  factor  of  safety  as  before,  the  work- 
ing tension  rises  to  2,612  Ibs.,  and  allowing  20  feet  deflection, 
the  possible  length  of  span  is 


L  = 


8  X26i2  X  20 


•4 

Spans  of  even  this  length  can  be  managed  without  any  very 
•elaborate  terminal  supports.  When  the  line  wires  are  heavy  and 
numerous,  or  longer  spans  must  be  used,  it  may  be  necessary 


476  ELECTRIC   TRANSMISSION  OF  POWER. 

to  use  stout  bearer  cables,  arranged  like  a  rudimentary  sus- 
pension bridge  with  a  footpath,  to  facilitate  inspection  and 
care  of  the  conductors.  The  expense  of  such  a  structure  is 
sometimes  justified  by  enabling  one  to  avoid  long  and  ex- 
pensive detours.  When  a  simple  long  span  of  conductors  is 
used,  the  support  of  the  ends  and  the  proper  insulation  of  the 
tense  wires  require  care.  A  timber  truss  well  guyed  will 
answer  in  most  cases,  and  the  strain  may  be  distributed  among 
several  stout  insulators.  The  conductors  should  always  be  in 
duplicate  across  such  a  span.  In  the  140  mile  transmission  of 
the  Bay  Counties  Power  Co.  to  Oakland,  Cal.,  an  extremely 
long  span  became  necessary  in  crossing  the  Straits  of  Carqui- 
nez.  Steel  towers  were  erected  on  high  ground  on  each  side 
of  the  straits,  and  the  conductors  are  four  -j  inch  steel  cables 
(one  being  in  reserve)  spanning  a  distance  of  4427  feet. 
These  cables  have  ultimately  to  take  current  at  60,000  volts 
and  are  carried  by  porcelain  and  micanite  insulators,  the  latter 
being  used  for  supporting  the  direct  pull  at  the  anchorages. 
(See  chapter  XVII.) 

When  bodies  of  water  too  wide  for  a  suspended  structure 
must  be  crossed,  there  is  trouble  ahead.  In  marshy  shallows 
a  timber  trestle  is  perhaps  the  best  way  out  of  the  difficulty, 
but  in  deeper  water  cables  may  occasionally  have  to  be  used, 
although  rarely  in  view  of  the  possibilities  of  very  long  spans 
like  the  one  just  mentioned. 

Cables  can  be  obtained  that  will  stand  2,000  to  3,000  volts 
alternating  current  with  a  fair  factor  of  safety.  Above  this 
pressure  success  is  problematical.  Near  the  ends  of  the  line 
before  the  raising  or  after  the  reducing  transformers,  cables 
may  be  successfully  used,  but  when  the  obstacle  is  in  the 
middle  of  a  long  line  the  choice  is  between  evils,  reducing  the 
pressure  locally  by  an  extra  transformation  or  going  the  long 
way  around.  Either  expedient  is  costly  and  to  be  avoided  if 
possible.  It  is  almost  needless  to  say  that  when  cables  are 
used  they  should  be  in  duplicate. 

POLES. 

As  a  rule  all  aerial  lines  in  this  country  are  carried  on 
wooden  poles.  Iron  poles  are  used  much  for  railroad  work, 


LINE   CONSTRUCTION. 


477 


and  abroad  considerably  for  miscellaneous  work,  including 
power  transmission.  They  are,  however,  not  to  be  recom- 
mended for  high  voltage  transmission  work,  as  they  give  alto- 
gether too  good  an  opportunity  for  troublesome  and  dangerous 
grounds.  The  ordinary  wooden  cross  arm  is  of  very  little 
value  as  an  insulator  at  high  voltages,  particularly  after  it  has 
weathered  for  some  time  and  is  dirty.  Its  resistance  in 
damp  weather  may  amount  to  only  two  or  three  thousand 
ohms  from  end  to  end,  and  such  a  cross  arm,  while  allowing 
severe  leakage  in  case  of  more  than  one  insulator  on  the  same 
cross  arm  breaking,  would  permit  a  highly  dangerous  ground 
on  an  iron  pole  in  case  of  a  fault  in  a  single  insulator.  An 
iron  pole  with  iron  cross  arm  is  infinitely  worse.  Moreover, 
iron  poles  cost  so  much  more  than  wooden  ones  that  the  dif- 
ference in  expense,  capitalized,  will  generally  much  more  than 
meet  the  depreciation  of  a  properly  built  wooden  pole  line, 
even  if  the  iron  poles  were  to  have  an  infinitely  long  life. 


TOTAL 
LENGTH  IN 
FEET. 

DIAMETER 
AT  TOP- 
INCHES. 

DIAMETER 

(/  FROM 

BUTT,  IN 
INCHES. 

DEPTH  OF 
SETTING. 

APPROXI- 
MATE 
WEIGHT,  IN 
POUNDS. 

NUMBER 
THAT  CAN 
BE  LOADED 
ON  A  PAIR 
OF  CARS. 

35 

7 

12^ 

5'  6" 

650 

90 

40 

7 

13 

6' 

900 

75 

45 

r/z 

14 

6'  6* 

1,000 

65 

50 

r/2 

16 

7' 

1,300 

50 

In  the  eastern  and  central  parts  of  the  United  States  white 
Northern  cedar,  chestnut,  and  Northern  pine  are  the  most 
desirable  woods  for  poles,  in  the  order  named.  West  of  the 
Rocky  Mountains  redwood  is  rather  a  favorite  and  stands 
even  ahead  of  cedar  in  estimation.  Abroad,  Norway  fir  is 
highly  valued. 

For  power  transmission  work  the  poles  should  be  both  long 
and  strong — long  to  carry  the  wires  well  out  of  reach  and 
often  above  other  circuits;  strong  to  stand  the  pressure  of  the 
often  heavy  wires  and  the  wind.  In  open  country  the  length 
is  less  important,  and  it  is  sometimes  well  to  use  rather  stubby 
poles,  say  not  over  35',  but  extra  stout.  The  poles  should  be 


ELECTRIC   TRANSMISSION  OF  POWER. 

straight  and  free  from  knots,  of  sound,  live  wood,  and  the 
bark  should  be  peeled  and  the  poles  trimmed  and  shaved. 

The  foregoing  table  gives  the  size  and  other  characteristics 
of  the  poles  most  likely  to  be  used  on  power  transmission 
work.  This  is  based  on  cedar  poles,  and  the  dimensions  given 
are  the  minimum  to  be  permitted  in  first-class  line  construc- 
tion. Pine  and  redwood  poles  are  of  about  the  weight  given, 
chestnut  is  nearly  half  again  as  heavy. 

It  will  be  noted  that  poles  of  these  lengths  have  generally  to 
be  carried  on  two  cars,  one  being  too  short.  Various  preserv- 
ative processes  are  used  to  increase  the  life  of  wooden  poles. 
Of  these  "  creosoting  "  is  generally  preferred.  The  process 
consists  of  stowing  the  poles  in  an  air-tight  iron  retort,  ex- 
hausting the  air,  and  with  it  withdrawing  the  sap  and  moisture, 
treating  with  dry  steam  for  several  hours,  and  then  forcing  in 
the  preservative  fluid,  often  crude  petroleum,  under  heavy 
hydraulic  pressure. 

When  not  specially  treated,  the  poles  should  be  coated 
heavily  with  pitch,  tar,  or  asphalt  on  the  portion  to  be  buried 
up  to  and  fairly  above  the  ground  level. 

The  pole  top  is  usually  wedgeshaped  or  pyramidal,  and  this 
roof  should  be  painted  or  tarred.  Before  the  pole  is  erected 
the  gains  for  the  cross  arms  are  cut  and  the  cross  arms  them- 
selves should  be  bolted  in  place  and  the  pins  set  for  the  insu- 
lators. The  upper  cross  arm  centre  should  be  10  to  18  inches 
below  the  extreme  apex  of  the  pole  and  the  lower  cross  arms  18 
to  36  inches  further  down.  In  power  transmission  work  employ- 
ing heavy  wires  the  spacing  of  the  cross  arms  should  be  guided 
by  the  arrangement  of  the  circuits,  there  being  no  standard 
practice. 

The  cross  arms  themselves  are  of  wood  having  the  same  char- 
acteristics of  strength  and  durability  as  the  poles;  hard  yellow 
pine  being  rather  a  favorite.  They  are  of  course  of  such 
length  as  the  work  demands;  in  power  work,  generally  from  4 
to  8  feet.  There  are  two  sectional  dimensions  in  common  use, 
4%"  X  3^",  and  4^"  x  3%".  The  latter  should  be  used  for 
the  longer  cross  arms  and  those  carrying  heavy  cables  or  the 
like,  while  the  former  serves  for  4'  or  5'  arms  not  heavily 
loaded.  The  cross  arms  are  best  secured  in  their  gains  by  a 
strong  iron  bolt  passing  through  both  the  pole  and  the  cross 


LINE   CONSTRUCTION.  479 

arms  in  a  hole  bored  to  fit  and  set  up  hard  with  wide  washers 
under  head  and  nut.  This  construction  makes  a  cleaner  job 
than  the  practice  of  fastening  the  cross  arm  with  two  lag 
screws,  and  permits  of  easier  changes  and  repairs.  The  bolt 
should  be  about  three-quarters  of  an  inch  in  diameter,  and  the 
gain  is  from  i"  to  2"  deep,  according  to  the  size  of  the  pole. 
Lag  screws  are  cheaper,  however,  and  are  as  a  rule  employed 
in  ordinary  work.  Cross  arms  6'  long  or  more  should  be  braced 
with  diagonal  iron  straps. 

In  ordinary  transmission  circuits  about  50  poles  per  mile 
should  be  used,  no  feet  apart,  or  48  per  mile,  being  a  com- 
mon spacing.  The  setting  should  be  carefully  done.  The 
earth  should  not  be  disturbed  more  than  enough  to  make  easy 
room  for  the  pole,  and  the  earth  and  gravel  filled  in  around  the 
pole  should  be  heavily  tamped.  When  setting  poles  in  soft 
ground  it  is  sometimes  impossible  to  give  them  stability  enough 
merely  by  tamping,  and  the  best  procedure  is  to  fill  in  concrete 
about  the  pole,  using  one  part  of  Portland  cement  to  three  or 
four  parts  of  sand  and  heavy  gravel  or  broken  stone. 

The  stresses  to  which  a  pole  line  is  exposed  may  be  classi- 
fied as  follows:  i.  The  direct  weight  of  the  wire  and  the 
downward  component  of  the  wire  tension.  2.  Bending  moment 
due  to  the  pull  of  the  wires  at  turns  in  the  line.  3.  Wind 
pressure  on  poles  and  wires.  4.  Wind  pressure  plus  ice. 

1.  In  power  transmission  lines  built  as  has  been  indicated 
the  crushing  stress  is  completely  negligible.     The  ultimate 
resistance  against   crushing  amounts  in  the  woods  used  for 
poles  to  at  least  5,000  Ibs.  per  square  inch.     The  ordinary 
pole  therefore  has  a  factor  of  safety  of  several  hundred,  and 
the  danger  of  crushing,  even  from  tense  and  ice-laden  wires, 
has  no  real  existence. 

2.  Bending  moment  is  more  serious,  since  the  forces  acting 
have  a  long  lever  arm.     The  ultimate  effect  of  this  stress  is 
to  break  the  pole,  generally  near  to  the  surface  of  the  ground, 
by  crushing  the  fibers  on  the  side  next  the  stress  and  pulling 
apart  those  on  the  other  side.     The  pull  or  push  necessary  to 
break  a  round  pole  by  bending  is  approximately 

P       A  SR  i£\ 

T^'    () 

where  A  is  the  area  of  the  pole  section  at  the  ground,  6*  the 


480  ELECTRIC   TRANSMISSION  OF  POWER. 

strength  per  unit  area,  R  the  radius  at  the  ground,  and  D  the 
distance  between  the  ground  and  the  centre  of  pressure. 

For  example,  take  a  40'  pole,  13"  in  diameter  at  the  ground. 
Taking  S  =  7,500  Ibs.  per  square  inch  and  the  centre  of 
pressure  as,  32'  above  the  ground,  (6)  becomes 

/>  =  '32  X  7,5oo  X  6.5  = 

4  X  12  X  32 

The  factor  of  safety  allowed  should  be  approximately  5.  Prac- 
tically, poles  at  angles  should  always  be  guyed,  like  terminal 
poles.  This  is  best  done  with  a  steel  rope  one-quarter  to  one- 
half  an  inch  in  diameter,  taken  from  as  near  the  centre  of'the 
stress  on  the  pole  top  as  the  position  of  the  circuits  permits. 
The  guy  rope  should  extend  downward  at  an  angle  of  from 
45°  to  60°  with  the  pole,  directly  back  from  the  direction  of 
the  pull  on  the  pole,  and  should  be  drawn  taut  and  securely 
fastened  to  a  tree  or  a  firmly  set  post.  Where  there  are  three 
or  four  cross  arms  what  is  known  as  a  Y guy  is  often  used,  con- 
sisting of  a  guy  rope  attached  near  the  pole  top  and  another 
just  below  the  cross  arms.  These  divide  the  tension  and  are 
moored  by  a  single  guy  rope  in  the  ordinary  manner.  This 
arrangement  is  not  commonly  needed  in  transmission  work 
save  when  the  circuits  are  numerous  or  the  strain  exception- 
ally severe,  and  in  any  case  great  care  should  be  taken  to 
keep  the  guy  wires  well  clear  of  the  high  voltage  lines.  Some- 
times two  or  more  light  guys  in  different  directions  are  val- 
uable in  securing  a  pole,  when  proper  setting  is  very  difficult, 
and  may  save  expensive  blasting. 

The    bending    moment    due     to     an     angle     is     normally 

2   T  cos  —  where  7*  is  the  tension  as  already  determined  and  a 
2 

is  the  angle  made  between  the  wires  at  the  turn.  For  the  simple 
circuit  of  No.  oo  wire  already  discussed  and  a  turn  with  120° 
between  the  wires,  taking  a  factor  of  safety  of  7  on  the  wire, 
the  tension  per  wire  is  507  Ibs.  The  total  pull  for  the  two  wires 
forming  the  circuit  is  then  2,028  Ibs.  X  cos  60°  =  1,014  Ibs., 
a  pressure  rather  greater  than  would  be  permissible  without 
guying. 

3.  The  wind  pressure  on  the  wires  has  already  been  com- 
puted, and  the  same  formula  serves  for  figuring  the  pressure 


LINE   CONSTRUCTION.  481 

on  the  poles,  using  the  mean  diameter  in  inches,  and  for  the 
total  pressure  multiplying  by  the  feet  of  pole  exposed.  For 
example,  assuming  a  pole  of  34'  out  of  ground,  7"  diameter  at  the 
top  and  13"  at  the  ground,  the  average  diameter  is  10",  and  for 
a  storm  giving  a  normal  wind  pressure  of  40  Ibs.  per  square 
foot, 

P  —  .05  X  40  X   10  X  34  =  080  Ibs. 

This  acts  virtually  at  the  middle  point  of  the  pole,  hence  it  is 
equivalent  to  340  Ibs  at  the  pole  top,  to  which  must  be  added 
the  pressure  on  the  wire  itself,  which  for  the  circuit  in  ques- 
tion amounts  to  about  145  Ibs.  more,  making  a  total  of  485  Ibs. 
This  is  well  within  the  safety  limit,  and  would  remain  so  even 
if  there  were  half  a  dozen  wires  instead  of  two.  As  40  Ibs. 
per  square  foot  is  an  extreme  wind  pressure,  never  met  in 
most  localities  at  all,  it  is  safe  to  say  that  a  well-set  line  of  the 
poles  assumed,  loaded  with  any  power  transmission  circuit 
likely  to  be  met  in  practice,  is  perfectly  secure  so  far  as  wind 
pressure  alone  is  concerned,  unless  the  line  is  literally  struck 
by  a  cyclone. 

4.  The  most  dangerous  stresses  on  an  aerial  line  come  from 
sleet  storms  that  load  the  wires  with  ice,  increasing  the  weight 
and  the  lateral  thrust  due  to  wind  pressure.  On  rare  occasions 
ice  may  be  formed  on  wires  to  the  depth  of  a  couple  of 
inches.  Such  a  coating  on  a  No.  oo  wire  would  weigh  about 
5.9  Ibs.  per  lineal  foot.  The  mere  weight  of  this  would  pro- 
duce a  tension,  assuming  d  =  2',  and  No.  oo  wire  as  before, 

,_       (ioo)a  X  6.3 

T  =  ±    8  x  2        =  4,ooo  (very  nearly), 

which  is  well  above  the  tensile  strength  of  the  wire  if  soft- 
drawn.  Allowing  a  wind  pressure  of  20  Ibs.  per  square  foot, 
the  pressure  on  a  single  span  of  100  feet  would  be 

P  —  .05  x  20  x  4  X  ioo  =  400  Ibs. 

Adding  to  this  170  Ibs.  pressure  on  the  pole  itself,  the  total  for 
a  single  circuit  of  2  wires  would  be  970  Ibs.  total  thrust,  which, 
while  high,  is  not  likely  to  carry  down  the  pole.  Even  6 
No.  oo  wires  would  give  a  total  thrust  of  only  2,570  Ibs., 
which  is  still  below  the  ultimate  strength  of  the  pole.  The  pole 


482  ELECTRIC   TRANSMISSION  OF  POWER. 

line  is  therefore  stronger  than  the  wires.  If  a  line  is  to  stand 
such  extreme  stresses,  which  are  far  beyond  really  practical 
requirements,  the  only  safe  plan  would  be  to  string  hard-drawn 
wire,  shorten  the  poles  and  increase  the  diameter,  and  guy 
frequently.  As  a  matter  of  fact  the  insulators  and  their  pins 
are  quite  sure  to  give  way  before  the  wires  or  poles  under 
these  extreme  stresses,  and  in  most  transmission  lines  are  the 
greatest  source  of  anxiety. 

The  insulators  themselves  can  be  made  strong  enough  to- 
stand  the  greatest  stresses  to  which  they  will  be  subjected, 
but  it  is  not  easy  to  so  support  them  as  to  give  ample  strength 
without  endangering  the  insulation.  The  ordinary  wooden 
pin  answers  well  if  the  circuits  are  not  very  heavy  or  likely  to- 
be  weighted  with  ice. 

By  common  consent  locust  is  the  material  best  suited  for 
pins,  which  for  general  line  work  are  about  12'  long  and  2"  in 
extreme  diameter  at  the  shoulder,  below  which  the  pin  is- 
cylindrical  and  1^2"  in  diameter.  This  fits  a  hole  bored  in  the 
cross  arm  and  is  secured  by  a  nail  driven  through  arm  and  pm. 
The  top  of  the  pin  is  threaded  for  the  insulator  to  be  used. 
Under  extreme  forces  these  pins  are  liable  to  break  at  the 
shoulder;  and  for  transmission  circuits  carrying  very  heavy 
wire,  for  long  spans  and  for  cases  where  special  insulators 
demand  extra  long  pins,  a  variation  of  this  construction  is 
desirable.  On  the  Pacific  coast  excellent  results  have  been 
obtained  from  encalyptus  pins,  which  are  even  tougher  and 
stronger  than  locust,  but  unfortunately  not  readily  obtainable 
in  the  East.  Lacking  both  locust  and  eucalyptus,  a  fair  pin 
may  be  made  from  seasoned  oak. 

Iron  or  steel  pins  have  been  tried,  but  while  they  have 
ample  strength  they  are  an  element  of  weakness  in  insulating 
very  high  voltages,  tending  to  work  and  grind  the  interior  of 
the  insulator  and  furnishing  an  interior  conductor  which  much 
enhances  the  danger  of  puncture. 

A  better  device  is  to.  make  a  heavy  pin  with  a  base  2"  or 
more  square  and  clamp  it  to  the  cross  arm  with  an  offset  iron 
strap  the  full  depth  of  the  cross  arm. 

In  ordinary  line  work  the  pins  are  set  12"  to  14"  between 
centres.  With  heavy  wires  this  distance  may  advantageously 
be  increased  to  18"  to  24".  At  very  high  voltage  these  dis- 


LINE   CONSTRUCTION.  485 

tances  must  be  increased  farther,  perhaps  up  to  36"  or  even  60", 
in  dealing  with  voltages  in  the  uncertain  region  beyond  50,000 
volts.  • 

When  the  lines  have  to  be  transposed,  as  in  long  parallel 
alternating  power  circuits,  this  transposition  involves  some 
careful  work,  for  the  wires  must  be  kept  well  clear  of  each 
other.  Heavy  strain  pins  clamped  as  described  will  generally 
answer  the  purpose  and  allow  the  transposition  to  be  safely 
made.  Such  transposition  should  not  be  made  at  an  angle  or 
elsewhere  where  the  tension  on  the  insulators  is  unusually 
great. 

A  good  example  of  line  construction  for  heavy  transmission 
work  is  found  in  the  line  constructed  a  few  years  ago  for  the 
Niagara-Buffalo  power  circuit.  Fig.  222  shows  the  pole  head. 
The  cedar  poles,  intended  ultimately  to  carry  12  cables  each 
of  350,000  c.  m.,  are  extra  heavy,  varying  from  35  to  50  feet 
in  length  with  tops  9"  and  10"  in  diameter.  The  two  main 
cross  arms  are  of  yellow  pine,  12'  long  and  4"  X  6"  in  section, 
fastened  to  the  pole  with  long  lag  screws,  and  braced  by  an 
angle  iron  diagonal  ^"  x  2^",  bolted  to  the  pole  and  to  the 
bottom  of  the  cross  arm  at  each  side.  Each  side  of  each  arm 
is  bored  for  three  pins  spaced  18"  apart.  The  transmission  is 
three-phase  and  one  complete  circuit  is  on  each  side  of  each 
cross  arm.  The  cross  arms  themselves  are  2'  apart. 

The  pins  and  insulators,  Fig.  223,  are  special,  the  pins  being 
much  heavier  than  usual  and  the  insulators  of  dense  porcelain 
formed  in  the  usual  double  petticoat  design.  They  have  one 
peculiar  feature:  a  gutter  is  formed  on  the  external  surface, 
leading  to  diametrically  opposite  lips  so  placed  as  to  shed 
dripping  water  clear  of  the  cross  arm,  thus  lessening  the  dan- 
ger of  ice  formations.  Each  of  the  main  circuits  is  designed  to 
transmit  5,000  HP.  A  short  cross  arm  below  the  others  carries 
a  private  telephone  line.  The  right  of  way  is  in  part  owned  by 
the  operating  company  and  fenced  in,  and  in  part  along  the 
Erie  canal.  The  line  is  elaborately  transposed  every  five 
poles  to  annul  induction.  So  frequent  transposition  is  un- 
usual and  generally  needless.  Transposition  every  20  to  40 
poles  is  ample  for  ordinary  cases  and  on  long  lines  in  open 
country  it  is  enough  to  transpose  once  in  a  couple  of  miles. 

This  line  is  admirably  constructed,  but  it  is  a  grave  question 


ELECTRIC  TRANSMISSION  OF  POWER. 


whether  all  the  circuits  should  be  carried  on  a  single  pole  line 
on  account  of  the  difficulty  of  executing  repairs,  and  the 
insulators  are  rather  closer  to  the  cross  arms  than  seems  safe 
in  view  of  the  climate  and  the  high  voltage  to  be  employed. 
Certainly  at  voltages  above  10,000  a  duplicate  pole  line  is 
preferable  to  running  two  circuits  on  one  pole  line.  It  is, 
however,  entirely  feasible  to  execute  repairs  on  one  side  of  a 
pole  like  Fig.  222  while  the  circuit  on  the  other  side  is  in  use, 
although  it  is  a  careful  job  and  should  not  be  attempted  un- 
less, as  in  this  case,  the  cross  arms  are  unusually  long. 


iHr-r 

i'     -T~ 


A 


--J 


tl 


iU 11 


LE 


ID 


FIG.  222. 

Sleet  is  greatly  to  be  feared  on  such  lines,  for  although  ice 
is  a  very  fair  insulator,  at  10,000  or  20,000  volts  it  is  not  diffi- 
cult to  start  enough  leakage  to  break  down  the  insulation  and 
very  likely  burn  off  a  cross  arm.  In  one  case  that  has  come 
to  the  author's  notice  a  tree  trunk  16"  in  diameter  was  burned 
entirely  off  by  a  wire  carrying  a  5,000  volt  alternating  current 
in  the  course  of  less  than  a  day. 

Security  is  absolutely  necessary  in  a  power  transmission 
line,  so  that  the  construction  must  be  calculated  and  executed 
with  particular  care.  There  are  many  useful  precautions  that 
can  be  taken,  such  as  guard  irons  to  prevent  wires  slipping 
from  the  cross  arms  when  an  insulator  breaks,  specially  braced 
poles  at  short  intervals  to  relieve  longitudinal  strains,  local  cut- 


LINE    CONSTRUCTION. 


485 


out  stations  to  facilitate  the  execution  of  repairs,  and  so  forth. 
Whatever  the  general  construction,  two  things  in  particular 
must  be  kept  constantly  in  mind:  The  insulation  of  the  line 
must  be  kept  up,  and  must  be  constantly  watched  and  tested. 
Ordinary  methods  of  testing  insulation  are  nearly  useless  on 
high  voltage  lines,  since  a  line  may  show  practically  perfect 
insulation  with  a  testing  battery  and  be  practically  grounded 
when  used  at  10,000  volts.  Experience  shows  that  high  volt- 


FIG.  223. 

age  lines  must  quite  generally  be  considered  as  grounded,  even 
aside  from  the  apparent  grounding  due  to  capacity.  On  high 
voltage  systems  means  should  be  provided  for  testing  the 
insulation  at  or  near  the  working  voltage.  This  involves  some 
trouble,  but  is  well  worth  it,  since,  if  the  insulation  is  faulty,  it  is 
apt  to  go  rapidly  from  bad  to  worse  and  to  end  in  an  interrup- 
tion of  service.  It  is  not  a  difficult  matter  to  construct  a  high 
resistance  bridge,  using  tubes  of  Hittorfs  cadmium  iodide 
solution,  and  whether  the  bridge  or  any  other  of  the  familiar 
methods  of  measuring  very  high  resistances  be  used,  it  is  not 
a  hard  task  to  make  a  testing  battery  even  for  high  voltage, 


486  ELECTRIC   TRANSMISSION  OF  POWER. 

using  storage  cells  made  out  of  test  tubes  and  U-shaped  bits 
of  lead  wire,  or  better,  to  test  with  alternating  current. 
*  Lines  should  be  tested  as  often  as  the  opportunity  offers, 
and  any  signs  of  weakness  in  the  insulation  followed  up  closely. 
High  voltage  lines  are  likely  to  develop  faults  very  quickly 
indeed  if  there  is  incipient  weakness  in  the  insulation. 

Another  matter  of  most  grave  importance  is  the  installa- 
tion and  maintenance  of  proper  defenses  against  the  effects  of 
lightning.  Now  and  then  a  plant  may  be  located  so  that  it  is. 
practically  safe  from  lightning,  but  in  most  localities  thunder- 
storms are  not  rare  during  a  portion  of  the  year  and  may  be 
the  source  of  frequent  disaster. 

There  are  two  distinct  classes  of  lightning  strokes  which 
may  affect  aerial  circuits.  The  first  is  a  direct  stroke  from 
the  flash  itself,  taking  the  line  as  the  last  minute  portion  of  its 
terrific  leap  to  earth.  The  other  is  secondary,  being  the 
induced  current  due  to  the  enormous  energy  of  the  primary 
discharge,  quite  apart  from  the  line  and  perhaps  striking  from 
cloud  to  cloud.  When  these  tremendous  electrostatic  stresses 
suddenly  change  by  a  discharge,  the  induced  discharges  along 
conductors  may  be  very  formidable. 

Akin  to  this  phenomenon  are  the  sometimes  very  great 
potential  differences  between  line  and  earth  produced  by  the 
readjustment  of  the  charges  on  earth  and  clouds  raising  the 
potential  of  the  insulated  line.  The  line  and  earth  will 
normally  be  at  about  the  same  potential  with  respect  to  a 
heavily  charged  cloud,  but  when  the  clouds  and  earth  are 
mutually  discharged  the  insulated  line  may  assume  a  very  high 
potential  with  respect  to  the  earth  and  produce  a  violent  dis- 
ruptive discharge,  quite  akin  to  lightning.  This  kind  of  static 
discharge  may  be  produced  when  there  is  no  storm  and  no 
indication  of  anything  unusual  in  the  atmospheric  conditions. 
It  may  be  trivial,  only  enough  to  give  small  sparks  or  a  smart 
shock,  or  it  may  be  highly  dangerous  and  very  serious  in  its 
results. 

The  direct  lightning  stroke  is  at  once  the  most  formidable 
and  the  least  common  of  these  forms.  It  may  strike  the  line 
fairly  or  merely  give  it  a  branch  discharge  of  greater  or  less 
magnitude.  Next  in  frequency  and  severity  comes  the  second- 
ary lightning  flash,  which  is  far  more  common  and  is  the  variety 


PLATE  XV. 


LINE  CONSTRUCTION.  487 

most  generally  observed  during  thunderstorms.  The  static 
flashes  last  mentioned  are  somewhat  difficult  to  distinguish 
from  the  secondary  flashes  and  are  probably  even  more  com- 
mon. They  make  up  the  bulk  of  the  line  disturbances  in  clear 
weather  or  coincident  with  distant  storms,  and  as  a  class  are 
less  to  be  feared  than  the  direct  and  secondary  strokes. 
Lightning  arresters,  so  called,  are  in  the  main  short  spark  gaps 
between  the  line  and  a  grounded  conductor,  furnishing  the 
discharge  an  easier  path  to  ground  than  going  via  the  power- 
house machinery  or  an  insulator  and  pole.  In  connection  with 
this  spark  gap  device  is  some  means  for  preventing  the  dynamo 
current  from  following  in  the  wake  of  the  lightning  and  pro- 
ducing a  short  circuit. 

One  of  the  best  known  lightning  arresters  for  high  voltage 
alternating  circuits  is  that  due  to  Mr.  Wurts,  and  very  gen- 
erally used  in  the  practice  of  the  Westinghouse  Co.  The  unit 
arrester  of  this  system  is  shown  in  Plate  XV,  Fig.  i.  It 
consists  of  seven  cylinders  of  the  so-called  "  non-arcing  "  alloy, 
having  finely  knurled  surfaces.  They  are  set  usually  about 
^z"  apart  in  a  porcelain  block  with  a  cover,  as  shown  in  the  cut. 
There  are  thus  six  spark  gaps  in  series  in  each  unit,  a  degree 
of  subdivision  which  tends  to  discourage  the  maintenance  of 
an  arc.  The  lightning  can  jump  these  gaps  easily,  but  the 
arc  which  follows  is  choked  and  broken  by  the  whiff  of  non- 
conducting oxide  thrown  off  in  its  passage,  and  generally 
without  even  scarring  the  cylinders. 

On  long  high  voltage  power  transmission  lines  the  very 
extent  of  wire  gives  an  added  chance  for  lightning  strokes, 
and  the  normally  high  voltage  requires  spark  gaps  aggregating 
a  considerable  space  to  prevent  a  discharge  to  earth  merely 
from  the  working  voltage.  It  therefore  becomes  necessary  to 
put  several  unit  arresters  in  series,  and  to  provide  some  means 
for  sending  the  lightning  to  earth.  As  lightning  is  really  a 
very  rapid  and  oscillatory  discharge  it  is  blocked  by  an  induct- 
ance to  a  far  greater  extent  than  the  slowly  periodic  line  cur- 
rent. Hence,  if  a  "  choke-coil"  is  put  in  circuit  between  the 
lightning  arrester  and  the  generator  the  tendency  is  to  check 
an  entering  lightning  discharge  and  divert  it  into  the  arrester 
while  only  slightly  impeding  the  working  current. 

This  combination  of  arrester  and  choke-coil  is  the  one  gen- 


488  ELECTRIC    TRANSMISSION  OF  POWER. 

erally  used  on  high  voltage  lines.  The  choke-coil  ordinarily 
employed  in  connection  with  the  Wurts  arrester  is  the  flat 
spiral  form  similar  to  Fig.  224,  which  impedes  the  lightning 
more,  and  the  working  current  less,  than  the  more  familiar 
shapes.  In  default  of  this,  a  coil  can  be  readily  extemporized 
from  rubber  covered  wire,  25  or  30  turns  being  wound  up  in  a 
coil  of  say  18"  diameter  and  firmly  taped  together.  For  further 
protection  it  has  sometimes  been  found  desirable  to  use  sev- 
eral sets  of  arresters  in  parallel  separated  by  choke-coils,  so 
that  the  lightning  will  have  to  force  a  way  through  several 
choke-coils  with  a  chance  to  go  to  earth  at  each  coil. 


FIG.  224. 

This  arrangement  of  lightning  arresters  is  for  the  most  part 
pretty  effective.  The  weak  point  in  the  protection  of  high  volt- 
age alternating  circuits  from  lightning  lies  in  the  very  consider- 
able striking  distance  of  the  working  voltage  itself  and  the  in- 
crease of  this  distance  by  static  phenomena,  minor  resonance 
and  other  causes  occurring  normally  and  frequently  on  the  cir- 
cuits, quite  apart  from  lightning  discharges.  If  the  aggregate 
spark  gap  of  the  arresters  is  large  enough  to  prevent  frequent 
discharges  due  to  these  ordinary  causes,  there  is  an  unpleas- 
antly good  opportunity  for  heavy  sparks  due  to  inductive 


LINE   CONSTRUCTION. 


489 


lightning  discharges  to  get  through  the  arresters  in  spite  of 
the  choking-coils  and  do  mischief  in  the  station.  It  is  one 
thing  to  provide  paths  to  earth  for  the  lightning,  and  quite 
another  to  make  it  follow  them.  If  the  spark  gaps  are  so  far 
reduced  as  to  let  through  with  considerable  certainty  any 
lightning  discharge  of  serious  magnitude,  they  are  also  likely 


Ground 


FIG.  225. 


to  frequently  let  through  casual  discharges  followed  by  arcs 
sufficient  to  injure  in  time  the  arresters  and  even  to  interfere 
with  the  service. 

To  avoid  these  difficulties  the  Westinghouse  Co.  has  re- 
cently adopted  as  its  standard  the  arrangement  shown  in  Fig. 
225,  as  arranged  for  a  25,000  volt  circuit.  It  consists  of  twen- 


49°  ELECTRIC  TRANSMISSION  OF  POWER.. 

ty-four  of  the  unit  arresters  in  series,  connected  to  the  line  by 
an  adjustable  auxilary  gap  and  earthed  through  a  moderate 
resistance.  It  is  installed  as  usual  with  a  choke-coil  between 
it  and  the  machines.  The  peculiarity  of  this  arrester  lies  in 
the  high  shunt  resistance  around  about  half  the  series  of 
arrester  units.  This  makes  the  arrester  more  sensitive,  so 
that  moderate  increase  of  potential  will  throw  a  discharge  to 
earth  through  the  series  gaps  and  the  high  resistance. 

It  is  claimed  that  a  really  heavy  discharge,  as  from  lightning, 
will  be  sufficiently  checked  by  the  shunt  resistance  to  drive  it 
across  the  shunted  gaps,  thus  giving  the  full  arc  breaking 
power  of  all  the  gaps  in  case  of  need.  Such  selective  action 
if  regularly  effected  in  practice  should  be  very  useful  in  pre- 
venting constant  action  of  the  arresters  on  too  small  provoca- 
tion, while  providing  sufficient  gaps  to  take  care  of  severe 
discharges.  The  series  resistance  shown  next  the  earth  is 
merely  to  moderate  the  violence  of  the  ground  when  the  arc 
tries  to  follow  the  lightning  discharge. 

The  auxiliary  gap  is  of  considerable  service  in  enabling  the 
spark  gap  resistance  to  be  carefully  adjusted  to  the  voltage  of 
the  line  so  that  a  small  increase  of  potential  will  be  at  once 
relieved.  As  one  turn  of  the  adjusting  head  changes  the  gap 
by  -fa",  the  necessary  regulation  can  be  quickly  and  easily 
effected.  Of  course  one  such  arrester  group  as  is  shown  in 
Fig.  225  must  be  installed  for  each  wire  of  the  circuit  to  be 
protected. 

For  convenience  these  composite  arresting  devices  are  made 
up  into  compact  panels  ready  to  be  attached  to  the  line  and  to 
earth.  Fig.  2,  Plate  XV,  shows  the  front  and  back  of  the 
panel  corresponding  to  the  arrangement  of  Fig.  225,  one  such 
panel  being  connected  in  each  line  of  the  25,000  volt  circuit. 

Arresters,  however  carefully  arranged,  do  not  always 
work  properly,  sometimes  failing  to  protect  the  machines 
and  sometimes  causing  tremendous  momentary  ground- 
ing of  the  lines  through  them.  If  the  aggregate  spark 
gap  is  kept  as  it  should  be  down  to  a  point  where  a  moderate 
increase  over  the  full  line  voltage  will  break  through  to  earth, 
it  is  generally  good  policy  not  to  make  the  path  to  earth  of 
so  low  resistance  as  to  cause  a  formidable  short-circuit  if  the 
arc  following  a  discharge  is  not  broken  promptly,  as  sometimes 


PLATE   XVL  ^  .„. 

" 


LINE   CONSTRUCTION. 


491 


happens.  Particularly  in  case  of  three-phase  lines  it  is  inadvis- 
able to  use  a  common  earth  connection  for  the  three  lines  un- 
protected by  resistance,  since  if  an  arc  starts  there  ensues  a 
short-circuit  between  two  phase-wires  which  may  be  very 
destructive.  Such  accidents  occur  even  with  the  most  care- 
fully installed  arresters. 

The  amount  of  resistance  needed  depends  somewhat  on  the 
size  of  the  machines  to  be  protected,  and  more  on  the  voltage 
of  the  lines.  If  too  great  it  tends  to  lessen  the  value  of  the 
apparatus  for  diverting  dangerous  lightning  discharges,  if  too 
small  it  gives  altogether  too  good  a  chance  for  starting  an  arc 
on  a  scale  that  the  arc-extinguishing  powers  of  the  arrester  can- 


FIG.  226. 

not  cope  with.  Roughly,  there  should  be  in  a  10,000  volt  plant 
several  hundred  ohms  between  phase  and  phase  if  the  condi- 
tions are  such  that  discharges  are  likely  to  be  frequent. 

The  whole  matter  needs  investigation.  Sometimes  a  plant 
protected  by  arresters  having  no  resistance  at  all  in  series 
with  the  spark  gaps,  will  experience  no  trouble  from  the  cause 
mentioned,  while  in  another  similar  plant  there  will  be  inces- 
sant difficulty  from  it. 

In  the  lightning  arresters  developed  by  the  General  Electric 
Co.  the  other  horn  of  the  dilemma  is  taken,  i.e.,  resistance  to 
a  considerable  amount  is  in  series  with  the  spark  gaps  of  each 
arrester  unit.  The  unit  arrester  for  high  voltage  alternating 
lines  is  shown  in  Fig.  i,  Plate  XVI,  and  the  same  cased  for 
outdoor  use  in  Fig.  2.  It  consists  of  three  brass  cylinders 
about  i^"  in  diameter  spaced  from  ^"  to  TV'  apart,  and  con- 
nected in  series  with  them  a  pair  of  carbon  resistance  rods, 
each  of  a  resistance  varying  according  to  the  case  for  which  it 
is  to  be  used,  from  less  than  100  to  several  hundred  ohms. 


492 


ELECTRIC  TRANSMISSION  OF  POWER. 


One  arrester  unit  is  customarily  used  for  each  2,000  volts. 
They  are  customarily  installed  with  choking  coils  between; 
them  and  the  machines  as  already  explained.  Typical  con- 
nections for  a  3,000  volt  single-phase  line  are  shown  in  Fig. 
226.  For  a  10,000  volt  line  four  or  five  of  the  units  shown 
would  be  put  in  series  for  each  high  voltage  line,  as  shown  in 
Fig.  227,  and  several  groups  in  series  separated  by  choking 
coils  should  preferably  be  used.  As  between  these  arresters 
and  those  previously  described  there  is  no  great  choice.  The 
writer  has  known  numerous  instances  in  which  each  form  has 
succeeded  and  each  has  failed,  and  instances,  too,  in  which 
each  has  succeeded  after  the  other  had  been  tried  and  had 
failed  dismally. 


FIG.  227. 

The  carbon  rods  used  for  resistance  are  sometimes  shivered 
by  a  severe  discharge,  and  those  of  liberal  cross  section  and 
very  moderate  resistance  are  to  be  preferred.  The  tendency 
here  is  to  use  rather  too  high  resistance,  just  as  contrariwise 
in  the  Wurts  arresters  there  sometimes  proves  to  be  altogether 
too  little  resistance.  The  general  arrangement  and  liberal 
use  of  choking  coils  in  the  latter  is,  however,  hard  to  improve 
upon. 

For  continuous  current  circuits  the  Thomson  magnetic  blow- 
out arresters  have  proved  remarkably  effective.  The  form 
ordinarily  used  for  arc  circuits  is  shown  in  Fig.  3,  Plate  XVI. 
It  consists  of  a  pair  of  curved  metallic  horns  located  between 
the  poles  of  a  small  but  heavily  wound  electromagnet  energized 
by  the  main  circuit.  The  line  is  connected  to  one  horn,  and 
the  ground  wire  to  the  other.  If  a  lightning  discharge  leaps 


LINE    CONSTRUCTION. 


493 


the  gap,  the  following  arc  is  at  once  repelled  to  the  tips  of  the 
horns  and  blown  out.  Choking  coils  may  be  advantageously 
used  in  connection  with  these  arresters,  although  the  reactance 
of  the  fields  of  an  arc  machine  generally  kicks  a  lightning  dis- 
charge to  ground  without  any  injurious  results.  The  con- 
nections of  these  arresters  to  an  ordinary  arc  circuit  are  shown 
in  Fig.  228. 

Whatever  arresters  are  used,  they  must  be  installed  where 
they  are  easily  accessible,  so  that  they  may  be  kept  clean  and 
inspected.  They  should  never  be  put  in  a  wire-tower,  or 
similar  place,  and  had  better  be  just  outside  the  dynamo  room 
than  in  it.  When  in  place  they  should  be  tested  and  adjusted 
to  fit  their  conditions.  The  number  of  spark  gaps  should  be 


Ground 


FIG.  228. 


reduced  experimentally  until  the  line  voltage  under  the  ordi- 
nary working  conditions  of  the  circuit  will  just  fail  to  provoke 
a  discharge.  Then  there  should  be  enough  resistance  inserted 
to  prevent  mischief  if  the  arc  follows  a  discharge.  A  few 
tests  with  light  fuses  in  series  with  the  gaps  will  generally  put 
one  on  the  right  track.  The  arresters  themselves  should  be 
overhauled  to  see  that  the  gaps  are  what  they  purport  to  be, 
and  an  approximately  correct  adjustment  can  be  made  by 
reference  to  the  sparking  distances  of  the  voltage  employed. 
The  Committee  on  Standardization  of  the  American  Institute 
of  Electrical  Engineers  reported  a  table  of  standard  sparking 


494 


ELECTRIC   TRANSMISSION  OF  POWER. 


distances  between  needle  points  which  is  shown  in  very  con- 
venient graphical  form  in  Fig.  229.  As  cylinders  should  show 
materially  smaller  sparking  distances  for  the  same  voltages 
than  points,  this  table  will  serve  as  an  approximate  guide  in 
adjusting  the  lightning  arresters.  If  the  circuit  shows  sparking 
distances  at  the  arresters  greater  than  those  of  the  table,  keep 
an  attentive  eye  out  for  resonance. 

Lightning  arresters  are  sometimes  used  as  electrical  safety 
valves  to  keep  excessive  voltages  off  the  circuits,  especially 
where  there  are  cables  to  be  protected.  For  example  they 


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are  sometimes  connected  across  the  terminals  of  a  constant 
current  alternating  transformer  for  series  arcs  and  in  similar 
situations  where  extraordinary  voltage  might  be  for  any  cause 
feared.  Their  use  in  such  cases  should  be  guided  by  the  same 
principles  as  in  installing  them  for  lightning  protection. 

Inductance  coils  must,  however,  be  used  with  caution  on  an 
alternating  circuit,  since,  they  add  to  a  general  inductance 
already  too  much  in  evidence  and,  if  sufficient  to  deflect  light- 
ning across  the  arresters  to  earth,  may,  when  used  too  fre- 
quently, seriously  increase  the  line  inductance. 

One  of  the  commonest  devices  to  lessen  the  danger  from 
lightning  is  a  line,  usually  of  barbed  fence  wire,  carried  on  the 
pole  top  and  grounded  at  frequent  intervals.  In  the  Niagara- 
Buffalo  circuit,  shown  in  Fig.  222,such  lines,  grounded  at  every 
fifth  pole,  are  carried  on  the  guard  irons  at  each  end  of  the 
upper  cross  arms. 


LINE  CONSTRUCTION.  495 

In  certain  cases  these  guard  lines  have  worked  well;  else- 
where they  have  proved  totally  useless.  A  line  of  barbed  wire 
strung  above  the  main  line  certainly  has  a  considerable  tend- 
ency to  keep  the  atmospheric  potential  equalized  in  its 
immediate  neighborhood,  and  thus  may  check  minor  static 
phenomena.  It  also  may  catch  certain  branch  discharges  from 
a  nearby  lightning  stroke,  and  ameliorate  minor  secondary 
discharges,  but  there  is  little  reason  to  expect  much  more 
than  this. 

The  guard  line  would  have  to  be  carried  well  above  the 
other  wires  to  be  of  much  use  as  a  lightning  rod  in  case  of  a 
direct  stroke,  and  is  powerless  to  prevent  secondary  strokes  or 
static  discharges  between  wire  and  earth,  although  it  may 
mitigate  their  severity.  In  regions  where  the  danger  of  direct 
lightning  strokes  is  not  very  great  and  the  discharges  mostly 
weak  such  a  guard  line  well  grounded  is  of  material  assistance, 
but  where  protection  is  needed  most  such  a  line  must  be  re- 
garded merely  as  a  precaution  of  possible  utility,  but  of  little 
value  unless  reinforced  by  the  best  lightning  arresters  that  can 
be  procured. 

So  far  as  the  station  is  concerned  the  best  procedure  is  to 
install  a  bank  of  lightning  arresters,  coupled  with  inductance 
coils  as  already  described,  preferably  putting  the  arresters 
outside  the  station  and  always  grounding  them  thoroughly. 
This  station  protection  must  be  kept  in  the  best  of  order,  for 
on  long  power  circuits  it  is  no  very  unusual  thing  to  see  the 
arresters  flash  several  hundred  tirn^s  in  a  single  storm  or  for, 
them  to  be  almost  continuously  in  action  for  some  minutes  at 
a  time,  and  with  all  this  use  the  arresters  must  not  break 
down. 

As  regards  the  line  there  is  no  complete  protection  avail- 
able, and  with  the  utmost  precautions  there  is  still  a  chance 
for  a  lightning  discharge  sufficient  to  break  down  the  insula- 
tion of  a  high  voltage  line.  About  the  most  that  can  be  done 
is  to  use  a  guard  line,  grounded  say  every  thousand  feet,  and 
to  protect  danger  points  with  special  lightning  arresters  like 
those  at  the  station.  Such  danger  points  may  be  found  on 
high  ridges  and  other  places  where  observation  shows  light- 
ning strokes  to  be  frequent,  and  particularly  the  terminals  of 
any  underground  or  submarine  cables,  banks  of  transformers 


49 6  ELECTRIC  TRANSMISSION  OF  POWER. 

and  other  apparatus  along  the  line.  If  the  line  is  in  sections 
with  section  boxes  for  testing,  cutting  out,  and  similar  work  at 
each  section  end,  these  are  the  places  for  lightning  arresters. 
The  author  most  emphatically  does  not  believe  in  the 
general  scattering  of  lightning  arresters  along  the  line  at 
every  few  poles.  If  the  line  is  at  10,000  or  more  volts  the 
arresters  are  likely  to  operate  on  very  small  provocation,  and 
numerous  arresters  mean  a  great  number  of  grounds  unless 
they  are  kept  in  the  best  of  order.  Therefore  the  arresters 
must  be  put  where  they  can  have  frequent  examination  and 
testing,  which  is  impracticable  if  they  are  scattered  promiscu- 
ously along  the  line.  It  must  not  be  forgotten  that  secondary 
distribution  systems  if  of  any  considerable  extent  are  very 
liable  to  trouble  from  lightning,  and  must  have  adequate  pro- 
tection. The  system  must  be  protected  as  a  whole  to  preserve 
continuity  of  service. 

Barring  cyclones  and  earthquakes,  an  aerial  line,  carefully 
designed  and  constructed,  is  exceedingly  durable  and  reliable. 
If  the  poles  are  creosoted  or  similarly  treated  they  should 
last  easily  twenty  or  twenty-five  years,  and  the  rest  of  the 
structure  is  nearly  as  durable,  while  bare  wire  does  not 
deteriorate  sensibly. 

A  poorly  set  line  of  cheap  material,  on  the  other  hand,  may 
be  unserviceable  in  less  than  half  this  time.  Here,  as  every- 
where else,  good  work  pays. 

A  well-nigh  indispensable  accessory  of  every  power  trans- 
mission line  is  a  private  telephone  line  connecting  the  power 
house  with  the  sub-stations  and  with  intermediate  points. 
Such  a  line  is  usually  carried  on  side  brackets  attached  to  the 
poles  six  or  eight  feet  below  the  power  wires.  This  line  is 
most  often  a  metallic  circuit  of  galvanized  iron  wire  about 
No.  12  in  size,  carried  on  ordinary  glass  insulators  and  trans- 
posed every  twenty  poles  or  so.  Such  lines  can  be  made  to 
give  fair  service,  but  the  transposition  of  the  wires  has  to  be 
very  carefully  adjusted  to  suppress  induction.  The  lengths 
of  wire  under  induction  must  agree,  not  within  a  few  poles 
merely,  but  within  a  few  feet,  to  avoid  annoying  singing.  The 
two  sets  of  insulators  should  be  kept  at  a  uniform  distance 
from  the  main  line  and  the  wires  should  be  drawn  uniformly 
tight  and  so  transposed  that  taking  the  line  from  end  to  end, 


LINE   CONSTRUCTION. 


497 


each  wire  shall  have  just  half  its  length  on  the  upper  and 
half  on  the  lower  bracket,  or  on  the  right  and  left  insulators 
if  a  short  cross  arm  is  used. 

The  wires,  too,  must  be  kept  clear  of  grounds,  from  foliage 
and  other  interference,  in  order  to  keep  the  inductive  balance 
perfect.  With  care  in  stringing,  the  line  can  easily  be  kept 


FIG.  230. 

in  good  operative  condition,  but  is  seldom  free  from  some 
residual  induction. 

A  far  better  although  considerably  more  expensive  line  is 
obtained  by  using  the  twin-wire  insulated  cable  made  for 
telephonic  purposes. 

On  long  lines  it  is  good  policy  to  make  provision,  say  every 
five  miles  or  so,  for  getting  at  the  high  voltage  line  for  repairs. 
If  the  line  is  in  duplicate  it  should  be  so  arranged  that  at 
these  junctions  jumpers  can  easily  be  put  on  between  wires 
in  the  same  phase  and  a  section  of  one  of  the  lines  cut  loose 


ELECTRIC   TRANSMISSION  OF  POWER. 

so  that  it  can  be  readily  handled.  At  such  points  there 
should  be  opportunity  for  cutting  in  a  portable  instrument  on 
the  telephone  line.  Telephone  boxes,  Fig.  230,  much  like  the 
ordinary  police  signal  box  can  be  obtained,  and  may  advanta- 
geously be  permanently  installed  at  the  ends  of  these  line 
sections.  These  are  good  points,  too,  for  installing  line  light- 
ning arresters  and  making  provisions  for  testing. 

The  commonest  accidents  on  high  voltage  lines  are  short 
circuits  from  branches  of  trees  and  broken  insulators.  The 
effect  of  the  first  is  to  start  an  arc  that  is  likely  to  burn  down 
the  line  if  the  branch  is  more  than  a  mere  twig.  There 
are  great  fluctuations  of  current  and  voltage,  and  the  charac- 
ter of  the  accident  is  generally  evident.  Broken  insulators 
may  in  dry  weather  produce  no  sensible  effect  at  all,  but  if 
the  cross  arms  are  damp  there  may  be  serious  leakage  between 
line  and  line  that  sometimes  ends  by  burning  up  the  cross  arm 
or  even  the  pole  top.  Broken  insulators  can  be  replaced  if 
necessary  iwhile  the  line  is  "alive,"  even  when  carrying 
pressures  as  high  as  15,000  or  even  20,000  volts.  The  line 
affected  can  be  pulled  or  pushed  clear  of  the  cross  arm  and 
held  clear  while  the  lineman  puts  on  a  new  insulator,  prefer, 
ably  one  with  a  top  groove  for  ease  of  manipulation.  Then 
the  line  can  be  pulled  back  into  position  and  an  insulated  tie 
wire  put  in  place,  if  needful,  with  long  rubber-handled  pliers, 
it  takes  a  skillful  and  cautious  lineman  to  do  the  job,  but  it 
can  be  done  if  necessary.  It  is  best  not  to  trust  to  rubber 
gloves  alone,  as  they  are  seldom  in  good  condition  and  there 
is  nearly  always  enough  leakage  around  the  pole  top  to  give 
a  powerful  shock.  Sometimes,  when  working  at  such  a  job, 
a  nail  is  driven  into  the  pole  well  below  the  workman  "and 
a  temporary  jumper  thrown  from  it  over  the  wire  under 
repair  so  that  the  lineman  will  be  less  likely  to  get  leakage 
shocks,  or  the  cross  arm  is  temporarily  grounded  by  a  wire 
for  the  same  purpose. 

Duplicate  lines  are  much  easier  to  repair,  since  one  can 
then  work  on  dead  wires,  and  for  very  high  voltages  duplicate 
pole  lines  are  better  still,  but,  with  care,  it  is  far  safer  and 
easier  to  work  on  high  voltage  lines  than  is  generally 
supposed. 


CHAPTER   XIV. 

CENTRES   OF    DISTRIBUTION. 

IN  many  cases  of  power  transmission  the  primary  object  is 
the  supply  of  power  and  light  in  various  proportions  through- 
out a  more  or  less  extended  region.  Therefore  the  question 
of  methods  of  distributing  electric  energy,  after  it  has  been 
received  from  the  transmission  line,  must  often  be  carefully 
considered.  The  subject  may  conveniently  be  treated  in 
three  divisions:  First,  distribution  direct  from  the  transmis- 
sion circuit  without  the  use  of  special  reducing  transformers 
or  sub-stations.  Second,  distribution  from  scattered  sub- 
stations. Third,  distribution  from  a  main  reducing  station. 
These  divisions  do  not  have  rigid  boundaries  and  often  over- 
lap, but  they  involve  three  quite  diverse  sets  of  conditions. 

Into  the  class  first  mentioned  fall  all  the  ordinary  electrical 
installations  wherein  the  power  station  is  separated  from  its 
load  by  a  transmission  line.  This  line  is  usually  of  moderate 
length,  for  otherwise  the  voltage  used  would  need  to  be 
reduced  for  the  working  circuit,  and  the  region  supplied  is 
generally  a  town  or  city  of  moderate  size.  Such  cases  are 
common  enough  and  generally  arise  from  the  existence  of  a 
convenient  water  power  half  a  dozen  miles,  more  or  less,  from 
a  town  that  needs  light  and  power,  or  that  has  already  a  cen- 
tral station  which  from  motives  of  economy  it  is  desirable  to 
operate  by  water  power.  The  power  is  therefore  developed  and 
new  distribution  lines  are  erected,  or  the  old  ones  reorganized. 
The  whole  condition  of  things  is  closely  similar  to  ordinary 
central  station  practice,  save  that  the  load  is  all  at  a  consider- 
able distance  from  the  station.  Only  the  use  of  alternating 
current  need  be  considered,  since  this  current  alone  is  practi- 
cally employed  for  general  purposes  at  distances  above  a  mile 
or  two. 

The  rudimentary  map,  Fig.  231,  gives  a  case  typical  of  many. 
The  power  station  is  at  A-9  with  a  line  across  country  to  the 

499 


5oo 


ELECTRIC   TRANSMISSION    OF  POWER. 


town  which  is  to  be  supplied  with  light  and  power.  The  dis- 
tance to  the  town,  A  B,  is  perhaps  four  miles.  Now  the  prob- 
lem is  to  distribute  the  energy  derived  from  A  over  the  town 
in  the  best  and  most  economical  way.  Since  much  lighting  as 
well  as  motor  service  is  to  be  done,  good  regulation  is  essen- 
tial, while  abundance  of  water  makes  small  variations  in  effi- 
ciency of  little  moment.  The  town  is  scattered,  with  a  main 
business  street,  C,  running  lengthwise  through  it. 

There  is  here  little  object  in  a  sub-station,  for  the  distances 
are  too  great  for  convenient  distribution  at  low  voltage,  and 
the  short  transmission  makes  it  desirable  to  avoid  raising  and 
reducing  transformers.  The  choice  of  a  system  is  the  first  con- 
sideration. This  is  not  a  question  of  such  vital  importance 
as  the  average  salesman  hastens  to  proclaim.  The  skillful 
organization  of  the  installation  will  make  much  more  differ- 


FIG.  231. 

ence  in  the  general  success  of  the  plant,  than  the  particular 
species  of  apparatus  used.  This  should,  however,  be  deter- 
mined with  due  regard  to  the  local  conditions. 

Any  alternating  system  except  plain  monophase  can  be 
made  to  do  excellent  service,  and  monophase  is  inapplicable 
only  in  default  of  suitable  small  motors,  which  are  not  at 
the  present  time  available  in  this  country,  at  least  in  any  form 
which  warrants  their  use  in  cases  where  motors  are  to  form 
anything  more  than  a  trivial  portion  of  the  total  load.  Some- 
thing depends  on  the  character  and  amount  of  the  motor 
service.  If  it  be  very  considerable  and  in  all  sorts  of  service, 
general  experience  both  in  this  country  and  abroad  indicates 
some  advantages  in  triphase  apparatus.  This  advantage,  how- 
ever, depends  more  on  the  ease  and  economy  with  which  a 
triphase  distribution  can  be  carried  out,  when  motors  and 


CENTRES  OF  DISTRIBUTION.  501 

lights  are  to  be  served  in  the  same  territory,  than  on  any  in- 
trinsic advantages  in  the  motors.  When  made  with  equal 
care  and  skill  all  polyphase  motors  are  substantially  alike  in 
their  properties.  Details  of  the  various  systems  of  distribu- 
tion will  be  given  in  treating  sub-station  work.  Where  the 
motor  service  consists  of  a  few  large  units,  even  the  mono- 
phase system  with  synchronous  motors  is  entirely  practicable, 
although  seldom  advisable.  Diphase  and  triphase  systems  can 
be  advantageously  applied  to  any  case  that  is  likely  to  arise; 
and  which  one  will  best  fit  it  is  a  matter  that  only  a  trained 
engineer  with  full  knowledge  of  the  local  conditions  can 
properly  decide. 

Of  far  more  importance  are  the  general  methods  employed  in 
carrying  out  the  electrical  distribution,  and  these  are  appli- 
cable with  almost  equal  force  to  any  sort  of  alternating  system. 

First  in  importance  is  the  maintenance  of  a  uniform  voltage 
•on  the  primary  service  lines.  This  voltage  should  as  far  as 
possible  be  the  same  at  every  transformer  and  should  be  con- 
stant, save  as  it  may  be  raised  to  compensate  for  the  loss  in 
the  secondaries. 

The  first  step  toward  obtaining  this  uniformity  is  to  assume 
a  fictitious  centre  of  distribution  as  at  Z>,  Fig.  231.  This 
should  be  chosen  at  or  near  the  centre  of  load,  generally  in  the 
business  centre  of  the  city.  If  the  office  of  the  operating  com- 
pany is  conveniently  situated  it  should  be  used  as  a  habitation 
for  the  centre  of  distribution,  at  which  supplies  can  be  kept 
and  measurements  made.  D  is  taken  as  the  termination  of 
the  transmission  line  proper  and  acts  in  the  capacity  of  a 
central  station  toward  the  primary  service  wires.  As  a  pre- 
liminary toward  a  more  exact  regulation,  there  must  be 
means  for  keeping  the  voltage  at  this  point  D  up  to  the 
normal  under  all  conditions  of  load.  The  most  obvious 
suggestion  is  overcompounding  the  generators  for  constant 
voltage  at  Z>,  and  this  is  often  advisable,  though  it  must  be 
remembered  that  compound  winding  is  by  no  means  the  only 
•and  not  always  the  best  means  of  securing  constant  voltage  at 
a  point  distant  from  the  generator. 

When  the  circuit  is  nearly  non-inductive,  and  the  current 
therefore  very  nearly  in  phase  with  the  E.  M.  F.,  or  when  the 
power  factor  can  be  kept  very  nearly  constant,  compounding 


502  ELECTRIC   TRANSMISSION  OF  POWER. 

works  admirably,  and  so  is  readily  applicable  to  cases  where 
lighting  is  the  main  work  to  be  done  or  where  synchronous 
motors  keep  up  the  power  factor  of  the  system. 

If,  however,  the  load  is  largely  of  induction  motors,  run- 
ning at  all  sorts  of  loads,  or  is  otherwise  of  strongly  induc- 
tive character,  compound  winding  alone  will  not  suffice  to 
keep  constant  voltage  at  the  point  D.  It  will  fail  in 
proportion  to  the  amount  of  compounding  necessary  to 
be  employed  and  for  two  reasons:  first,  because  of  the  direct 
effect  of  the  lagging  current  on  the  excitation  necessary;  and 
second,  because,  as  has  already  been  pointed  out  in  Chapter 
VI.,  the  lagging  in  phase  of  the  current  disturbs  the  functions 
of  the  commutator.  It  is,  therefore,  desirable  to  bring  "pres- 
sure wires  "  back  from  D  to  show  at  the  station  exactly  the 
condition  of  things  at  the  load,  so  that  the  voltage  may  be 
maintained  by  hand  regulation,  if  necessary.  This  is  of  course 
a  temporary  expedient  with  a  compound-wound  machine,  but 
it  may  avert  frequent  bad  service.  The  pressure  wires  may 
come  either  from  the  primary  circuit  at  the  centre  of  distribu- 
tion or  from  some  point  of  the  secondary  system  which  is 
chosen  to  represent  average  conditions  of  load.  The  latter  is 
the  preferable  method,  if  there  is  a  fairly  complete  system  of 
secondary  mains.  The  pressure  wires  may  be  taken  as  a 
guide  for  close  hand  regulation  or  may  operate  some  form  of 
automatic  control  of  the  field  rheostat.  Neither  hand  nor 
automatic  control  is  very  satisfactory,  if  the  generator  requires 
great  change  of  excitation  under  change  of  load.  For  the 
class  of  power  transmission  under  consideration  it  is  therefore 
better  to  use  a  generator  of  moderate  inductance  and  armature 
reaction,  whether  it  be  compounded  or  otherwise  regulated. 

For  the  pressure  wires  may  be  substituted  a  compensated 
voltmeter,  arranged  to  take  account  of  the  drop  in  the  line 
and  show  at  the  station  the  real  voltage  at  the  centre  of  dis- 
tribution. 

Granted  now  that  means  are  taken  to  regulate  the  voltage 
at  D  as  it  would  be  regulated  if  the  generator  were  at  that 
point,  the  distribution  problem  is  the  same  as  that  in  an  ordi- 
nary central  station.  Most  alternating  stations,  however,  are 
far  from  well  organized  in  this  respect.  Nothing  is  at  present 
commoner  than  to  find  an  alternating  station  which  receives 


CENTRES  OF  DISTRIBUTION. 


503 


pay  for  not  more  than  one-half  of  the  energy  delivered  to  the 
lines,  and  sometimes  this  low  figure  falls  to  one-third  or  even 
a  quarter.  This  unhappy  state  of  things  is  due  mainly  to 
badly  planned  secondary  circuits  and  to  the  indiscriminate 
use  and  abuse  of  small  transformers.  The  alternating  current 
transformer  is  a  marvellously  efficient  and  trustworthy  piece 
of  apparatus,  and,  perhaps  in  part  for  this  very  reason,  it  has 
been  often  the  victim  of  wholesale  misuse.  Without  going  in 
detail  into  the  case  of  sub-station  vs.  house-to-house  distribu- 
tion, it  is  sufficient  to  say  that  the  essential  thing  for  efficiency 
is  to  keep  the  transformers  in  use  well  loaded  and  hence  at 
their  best  efficiency,  and  that  for  this  purpose  a  few  large 


_J 


r 


FIG.  232. 

transformers  are,  on  the  whole,  much  better  than  many  small 
ones.  The  reason  for  this  may  be  best  shown  by  taking  the 
following  practical  example: 

A  given  region  requires,  let  us  say,  250  incandescent  lamps 
or  thereabouts,  together  with  fan  motors  and  perhaps  an  occa- 
sional large  motor.  These  are  distributed  among  a  score  of 
customers  scattered  over  a  couple  of  blocks,  Fig.  232.  The 
blocks  are  say  200  feet  long,  with  alleys  cutting  them  in  two. 
Now  these  customers  may  be  supplied  from  individual  trans- 
formers, or  all  may  be  supplied  from  one  transformer.  In 
either  case  the  lines  should  be  carried  in  the  alley.  In  the 
former  case  20  transformers  would  be  connected  to  service 
wires  attached  to  the  primary  service  main  a  b.  These 
transformers  would  average  say  12  lights  capacity  each  (600 


504 


ELECTRIC   TRANSMISSION  OF  POWER. 


watts).  In  the  latter  case  a  b  would  be  a  secondary  main 
supplied  from  a  single  transformer  of  12,000  watts  capacity. 
Now  assuming  a  load  such  as  would  be  met  in  ordinary  prac- 
tice, let  us  examine  the  transformer  losses  in  each  case.  The 
day  may  conveniently  be  divided  into  three  periods  in  con- 
sidering load:  7  A.  M.  to  5  p.  M.  forms  the  day  load  of 
motors  and  a  few  lights;  5  p.  M.  to  12  night,  the  evening  load; 
and  12  to  7  A.  M.,  the  morning  load.  During  the  first  period 
we  may  assume  15  transformers  to  be  quite  unloaded,  2  to  be 
three-quarters  loaded  on  motor  work  except  during  the  noon 
hour,  and  3  transformers  to  be  one-quarter  loaded  on  day 
lights. 
During  the  second  period  we  will  assume  the  motors  to  be 


u 

|5 


2  00  3  00 

FIG.  233. 


400 


500 


600 


off,  8  transformers  to  be  three-quarters  loaded  on  the  average 
from  5  until  7  p.  M.,  and  the  rest  one-quarter  loaded  from  5 
until  midnight.  • 

For  the  third  period  it  is  safe  to  assume  15  transformers  to 
be  unloaded  and  the  other  5  one-sixth  loaded  from  mid- 
night until  7. 

Now  the  efficiency  curve  of  a  500  or  600  watt  transformer  at 
various  loads  is  approximately  as  shown  in  Fig.  233,  derived 
from  a  .consideration  of  several  transformers  of  different 
makes.  The  constant  loss,  when  the  transformer  is  run 
unloaded,  is  about  30  watts. 

On  the  atove  assumptions  and  knowing  the  efficiency  of  the 


CENTRES  OF  DISTRIBUTION.  5°5 

transformer  at  various  loads,  it  is  easy  to  calculate  for  each 
period  the  total  energy  supplied  and  the  transformer  output 
which  is  delivered  and  paid  for.  The  result  of  this  calculation 

is  as  follows : 

1st  Period.  2d  Period.  3d  Period.  Total. 

Energy  Supplied  in  WattrWours,     18,480  20,420  10,050  48,950 

Energy  Delivered         "         "           10,450  17,700  3,5OO  31,650 

Therefore  barely  six-tenths  of  the  energy  supplied  to  the 
transformers  is  delivered  by  them  to  the  consumers.  And 
this  is  a  condition  of  things  more  favorable  than  is  usually 
found  in  stations  of  moderate  size  using,  as  many  of  them  do, 
small  transformers. 

The  other  method  of  distribution  is  to  use  a  single  large 
transformer  in  place  of  the  small  ones  and  distribute  to 
all  the  district  by  secondary  mains. 

Now  the  efficiency  of  a  10-12  KW  transformer  is  very 
closely  that  shown  in  Fig  234.  Moreover  the  energy  consumed 
when  running  without  load  is  hardly  more  than  150  watts,  so 
that  the  transformer,  when  absolutely  unloaded,  wastes  only 
one-fifth  of  the  energy  wasted  by  the  small  transformers  of  the 
same  total  capacity.  Taking  the  output  for  the  same  periods 
as  before,  a  much  better  result  is  reached,  as  follows: 

ist  Period.      2d- Period.     3d- Period.      Total. 

Energy  Supplied,  Watt  Hours,        nrfioo  19,^60  5, #30         37,290 

Energy  Delivered.     "        "  10,450  17,700  3,5<X>         31,650 

With  the  single  large  transformer  more  than  .80  per  cent, 
of  the  energy  supplied  to  it  is  delivered  on  the  customers'  cir- 
cuits. This  means  that  for  a  given  amount  of  energy  supplied 
from  the  station  one-third  more  revenue  will  be  obtained  if 
the  distribution  be  accomplished  by  a  large  transformer  as 
against  quite  small  ones.  Such  a  difference  is  important,  even 
in  a  plant  driven  by  cheap  water  power.  Besides,  for  a  given 
amount  of  energy  delivered  to  the  customers,  liigh  plant 
efficiency  means  smaller  first  cost  of  plant.  With  distribution 
by  secondary  mains  not  only  will  smaller  dynamos  at  the 
power  station  suffice  for  the  work,  but  the  cost  of  the  trans- 
former capacity  necessary  is  enormously  reduced.  In  the 
house-to-house  distribution  it  is  quite  possible  for  any  trans- 
former to  be  loaded  with  all  the  lights  connected  to  it.  When 
twenty  customers  are  supplied  from  a  single  transformer  the 


506 


ELECTRIC   TRANSMISSION  OF  POWER. 


chance  of  such  an  occurrence  is  almost  nil.  In  the  hypotheti- 
cal case  just  discussed  certain  of  the  transformers  would  be 
called  on  for  full  output  almost  daily,  while  all  of  them  would 
be  subject  to  such  a  demand.  The  largest  total  regular  out- 
put, however,  would  be  not  much  over  one-half  the  aggregate 
transformer  capacity.  So,  instead  of  using  a  12  KW  trans- 


o 

>    <*> 
o 

Z 
(J 


u 


u> 

KW     0 


45       6       7 
FIG.  234. 


e     9     10    ii    IE 


former  to  replace  20  small  ones,  in  reality  a  smaller  one,  say 
one  of  10  KW,  would  be  ample. 

In  point  of  cost  the  single  transformer  would  have  the 
advantage  by  not  less  than  $250,  enough  in  most  cases  to 
pay  for  the  secondary  mains.  In  regulation,  too,  the  single 
transformer  has  the  advantage,  for  the  load  is  less  liable  to 
sudden  fluctuations,  and  the  transformer  itself  regulates  more 
closely. 

In  practice  it  is  best  to  go  a  step  further  than  shown  in 
Fig.  232  and  connect  the  secondary  mains  at  a  and  If  to  the  next 


CENTRES  OF  DISTRIBUTION.  5°7 

section  of  secondary  just  across  the  street,  and  also  c  with 
the  main  in  the  next  alley,  so  as  to  form,  at  least  in  the  region 
of  dense  load,  a  complete  secondary  network.  Thus  each 
transformer  can  help  out  its  neighbor,  in  case  of  need.  The 
secondary  mains  should,  in  so  far  as  is  practicable,  be  designed 
for  the  same  loss  of  voltage,  and  the  compounding  and  other 
regulation  applied  to  the  generator  should  be  arranged  to 
compensate  for  the  loss  of  voltage  in  the  transformers,  and 
to  hold  the  voltage  as  steady  as  possible  in  their  secondary 
mains.  The  perfection  of  such  regulating  arrangements 
depends  of  course  on  the  uniformity  of  the  distribution  of 
load,  but  with  a  little  tact  in  arranging  the  circuits  variations 
in  voltage  at  the  lamps  can  often  be  kept  within  2  per  cent, 
of  the  normal  pressure.  In  large  systems,  as  will  be  pres- 
ently shown,  even  better  work  can  be  done. 

An  essential  point  in  the  use  of  secondary  mains  is  the  em- 
ployment of  fairly  high  voltage.  The  general  law,  that  the 
•amount  of  copper  necessary  in  a  given  distribution  varies 
inversely  as  the  square  of  the  voltage,  applies  here  with  great 
force. 

In  the  early  stages  of  alternating  work,  when  small  trans- 
formers were  nearly  always  used  and  regulation  was  gener- 
ally bad,  the  favorite  voltage  for  incandescent  lamps  was  about 
50  volts.  The  main  reason  for  continuing  this  practice  was 
the  fact  that  it  is  not  difficult  to  make  a  50  volt  lamp  that  will 
stand  much  abuse  in  the  way  of  varying  voltage.  With  good 
regulation  this  pressure  can  now  be  more  than  doubled  with 
equal  security  from  breakage  and  great  advantage  to  the  dis- 
tributing system.  Not  less  than  no  volts  should  be  used,  and 
a  pressure  of  115  to  120  volts  is  better,  as  it  gives  equally 
good  service  with  a  quarter  less  weight  of  copper.  From  the 
present  outlook  even  higher  voltage  is  becoming  practicable. 

It  is  not  always  advisable  to  do  all  the  work  of  distribution 
by  secondary  mains.  In  districts  where  the  service  is  scat- 
tered a  few  small  transformers  of  various  sizes  can  be  very 
advantageously  used,  but  should  be  generally  employed  as 
a  temporary  expedient  only,  and  shifted  to  another  field  of 
usefulness  when  the  service  grows  heavy  enough  or  stable 
enough  to  justify  installing  secondary  mains. 

Recurring  now  to  Fig.  231,  we  have  found  that  the  best  pro- 


508  'ELECTRIC   TRANSMISSION  OF  POWER. 

cedure  is  to  use  an  alternating  system,  compounded  or  other- 
wise regulated  so  as  to  hold  the  voltage  as  nearly  as  possible 
constant  at  the  secondary  terminals  of  the  transformers. 
These  should  be  large  enough  to  do  all  the  work  within  a 
distance  of  200  feet  more  "  or  less  and  should  feed  sec- 
ondary mains  at  a  pressure  of  say  115  volts.  When  these 
mains  are  more  than  usually  long  it  is  best  not  to  feed  current 
directly  into  them  but  to  employ  feeders  connecting  for 
instance  c,  Fig.  232  with  points  midway  between  c  and  a,  and 
c  and  £,  respectively.  Neighboring  secondaries  may  often  be 
interconnected  with  great  advantage. 

As  to  the  primary  distribution  we  have  assumed  a  centre 
at  Z>,  Fig.  231.  From  this  point  feeders  should  extend  to 
primary  mains  connecting  the  transformers  more  or  less  com- 
pletely, preserving  nearly  equal  drop  in  voltage  from  D  to 
each  transformer.  The  degree  of  elaboration  in  this  primary 
network  is  a  matter  to  be  determined  by  local  conditions.  If, 
for  example,  the  plant  is  of  rather  small  size  and  the  drop 
from  B  to  C,  Fig.  230  is  not  above  i  or  2  per  cent.,  the  trans- 
formers may  be  connected  to  short  branch  lines  crossing 
B  D  C  at  various  points,  without  any  further  complications, 
or  the  main  line  may  be  branched  at  B,  each  branch  having 
short  cross  feeders,  while  with  other  distributions  of  load  the 
primary  lines  may  be  quite  completely  netted,  with  regular 
feeders  from  D. 

The  motor  service  may  often  require  special  treatment.  It 
often  happens  that  it  is  best  to  feed  large  single  motors  or 
groups  of  motors  from  special  transformers,  which  will  gener- 
ally be  large  enough  to  avoid  the  objections  adduced  against 
a  general  house-to-house  transformer  system.  Such  special 
transformers  avoid  throwing  a  large  and  varying  load  on  the 
secondary  lighting  mains  during  the  hours  of  "lap-load" 
when  it  might  be  objectionable,  and  thereby  avoid  needlessly 
heavy  mains  and  annoying  variations  of  voltage. 

It  must  be  remembered  that  Ohm's  law  is  a  very  stubborn 
fact.  Any  apparatus  that  takes  a  large  and  variable  current  is 
liable  to  interfere  with  regulation.  There  is  no  such  thing  as 
a  motor  either  for  continuous  or  alternating  currents  which 
will  not  affect  the  lighting  service.  The  nearest  approach  to 
such  a  motor  is  obtained  by  arranging  the  distributing  system 


CENTRES  OF  DISTRIBUTION.  5°9 

so  that  the  largest  current  taken  by  the  motor  will  be  insuf- 
ficient noticeably  to  disturb  the  regulation  of  the  lamps. 

This  means  that  care  should  be  taken,  in  arranging  the  dis- 
tribution, to  avoid  overloading  the  lighting  mains  with  motors. 
It  is  an  easy  matter  to  determine  the  effect  of  the  motor  cur- 
rent by  calculation  if  the  current  is  continuous,  and  by  experi- 
ment or  calculation  for  alternating  current.  In  the  latter 
case  the  easiest  way  is  to  connect  the  motor  with  any  convenient 
main  and  put  on  load  with  a  brake — even  a  plank  held  against 
the  pulley  will  do.  Put  an  ammeter  in  circuit,  and  if  at  the 
rated  amperage  of  the  motor  the  fall  in  volts  at  the  transformer 
is  enough  to  endanger  regulation,  the  motor  should  be  put  on 
transformers  of  its  own.  Generally  the  likelihood  of  trouble 
can  be  judged  from  the  size  of  the  motor  and  the  load  on  the 
mains,  without  experiment.  One  of  the  advantages  of  regula- 
tion by  secondary  pressure  wires  is  the  easier  handling  of  an 
inductive  load  of  which  compounding  alone  generally  takes  no 
account. 

One  of  the  nice  questions  to  be  decided,  in  such  a  plant  as  is 
under  discussion,  is  arc  lighting.  The  most  obvious  method 
of  arc  lighting  from  a  transmission  plant  is  to  use  alternating 
motors  to  drive  arc  dynamos,  either  belted  or  directly-coupled. 
This  method  is  in  use  in  a  good  many  plants,  and  works  ad- 
mirably, although  the  efficiency  is  not  all  that  could  be  desired, 
being  probably  about  75  per  cent,  at  full  load,  reckon- 
ing from  the  energy  received  by  the  motor  to  that  delivered 
at  the  lamps  under  the  most  favorable  commercial  conditions. 
That  is,  for  the  operation  of  each  450  watt  (nominal  2000  c.  p.) 
continuous  current  arc,  about  600  watts  would  have  to  be 
delivered  to  the  motor.  In  working  commercial  circuits,  on 
which  the  number  of  lights  varies  greatly,  the  efficiency  at 
light  loads  would  be  greatly  reduced,  and  might  easily  fall  to 
between  50  and  60  per  cent. 

This  is  not  a  cheerful  showing,  and  much  ingenuity  has  been 
spent  in  attempting  to  remedy  such  a  state  of  things.  For 
street  lighting  the  scheme  is  reasonably  good,  but  it  breaks 
down  in  commercial  lighting. 

In  cases  where  plants  operate  low  tension  direct  current 
systems  via  motor-generator  or  rotary  converters,  the  solu- 
tion of  the  difficulty  is  simple,  since  all  the  commercial  arcs 


5io 


ELECTRIC   TRANSMISSION  OF  POWER. 


can  readily  be  worked  at  constant  potential,  using  preferably 
enclosed  arc  lamps  taking  from  5  to  7  amperes.  The 
efficiency  of  the  rotaries  is  high  and  the  loss  in  the  mains  is 
not  great,  so  that  by  this  means  the  only  circuits  that  need  be 
worked  on  the  series  system  are  the  street  lights,  which  form 
a  nearly  constant  full  load.  When  the  distributing  system  is 
alternating,  one  can  still  use  constant  potential  lamps  for  the 
commercial  circuits  with  fairly  good  results. 

Alternating  constant  potential  enclosed  arc  lamps  have  at 
the  present  time  been  brought  to  a  state  that  justifies  their 
extensive  use,  and  yet  it  must  be  admitted  that  they  are  some- 
what less  satisfactory  than  the  direct  current  arcs.  Taking 
lamps  as  they  are  found  commercially,  and  comparing  direct 
current  with  alternating  current  enclosed  constant  potential 
arc  lamps,  the  following  results  were  obtained  by  a  committee 
of  the  National  Electric  Light  Association  appointed  to  deal 
with  arc  photometry: 


Current.  -_ 

d 

il 

0^ 

H 

MEAN 
SPHERICAL  c.  p. 

WATTS 

PER  M.  S.  C.  P. 

Opal 
Globe. 

Clear 
Globe. 

Opal 
Globe 

Clear 
Globe 

Direct, 

4.90 

529 

155 

182 

3-41 

2.90 

Average  of 
8  lamps. 

Alternating, 

6.29 

417 

"4 

140 

3-66 

2.98 

Average  of 
7  lamps. 

These  figures  show  that  even  considering  the  energy  abso- 
lutely wasted  in  dead  resistance  in  the  direct  current  constant 
potential  lamp,  it  still  has  a  slight  advantage  in  efficiency  over 
the  alternating  lamp,  not  enough,  however,  to  compensate  in 
addition  for  the  loss  of  energy  incurred  in  changing  from 
alternating  to  direct  current. 

The  alternating  lamp  is  at  a  slight  further  disadvantage  in 
that  it  requires  rather  the  more  careful  handling,  and  a  rather 
better  grade  of  carbons.  It  also  is  liable  to  give  trouble  from 
noise,  although  the  best  recent  lamps  are  comparatively  free 
from  this  defect,  which  has  in  the  past  been  a  serious  objec- 
tion to  this  type  of  lamp.  Some  noisy  lamps  are  still  to  be 
found  on  the  market,  and  a  hard  carbon  will  make  almost  any 
lamp  sing.  Most  of  the  noise  originates  in  the  arc  itself,  and 


CENTRES  O.F  DISTRIBUTION.  ,:         511 

it  is  considerably  reduced  by  enclosing  the  arc  and  using  a 
non-resonant  gasket  on  the  outer  globe.  Good  lamps  care- 
fully operated  are  capable  of  giving  very  excellent  service, 
and  are  entirely  adequate  for  commercial  circuits  under  ordi- 
nary circumstances. 

It  would  appear  at  the  first  glance  at  the  table  just  given 
that  enclosed  arcs  of  either  type  are  little,  if  at  all,  more  efficient 
than  good  incandescent  lamps. 

The  difference  between  them  is  in  fact  not  very  great,  but 
incandescent  lamps  are  generally  rated  on  horizontal  candle 
power  and  on  their  initial,  not  their  average,  efficiency.  With 
due  allowance  for  this,  the  efficiency  of  the  best  commercial 
incandescent  lamps  per  mean  spherical  candle  power  ranges 
from  4  to  4.5  watts  per  candle  power,  so  that  the  arcs  have 
still  a  fair  margin  of  advantage,  increased  by  the  better  color 
of  their  light.  In  using  alternating  arcs  for  commercial  work 
their  performance  is  bettered  by  pushing  the  current  up  to 
about  7.5  amperes,  at  which  point  the  lamp  is  more  efficient, 
more  powerful,  and,  if  properly  adjusted,  steadier. 

In  street  lighting  the  best  results  have  so  far  been  obtained 
by  operating  alternating  arcs  in  series  on  the  constant  current 
system.  This  requires  special  lamp  mechanism  and  special 
devices  for  changing  the  constant  potential  alternating  current 
of  the  transmission  line  to  a  constant  alternating  current  of 
voltage  automatically  varying  with  the  requirements  of  the 
circuit. 

Such  constant  current  regulators  vary  considerably  in  de- 
tail, but  the  underlying  principle  of  all  of  them  is  as  follows: 
A  heavy  laminated  iron  core  is  surrounded  by  a  movable, 
counterbalanced  coil,  through  which  the  current  to  be  regu- 
lated flows.  Any  change  in  the  position  of  this  coil  changes 
the  reactance  of  the  combination,  and  hence  varies  the  current. 
The  coil  is  counterbalanced,  until,  when  the  normal  current  is 
flowing,  the  coil  floats  in  equilibrium,  the  opposing  forces 
being  gravity  and  the  attraction  or  repulsion  between  the  coil 
and  the  magnetized  core.  Then  any  change  of  current  due 
to  varying  conditions  in  the  external  circuit  establishes  a  new 
position  of  equilibrium  for  the  coil  in  which  the*  changed 
reactance  brings  the  current  back  to  its  normal  amount. 
Sometimes  the  regulator  is  combined  with  a  transformer  which 


512  ELECTRIC   TRANSMISSION  OF  POWER. 

receives  current  from  the  transmission  system  at  any  con- 
venient voltage,  while  the  floating  coil  acts  as  a  secondary 
and  delivers  constant  current  to  the  lighting  circuits. 

This  is  the,,  arrangement  of  the  constant  current  trans- 
former system  as  operated  by  the  General  Electric  Company. 
Fig.  i,  PI.  XVII,  shows  the  internal  arrangements  of  the 
transformer.  It  has  two  fixed  primary  coils  at  the  top  and 
bottom  of  the  structure,  receiving  current  from  the  main  line, 
and  two  floating  secondary  coils  counterbalanced  against  the 
repulsion  of  the  primaries,  and  balanced  against  each  other 
by  the  double  system  of  rocker  arms  visible  at  the  top  of  the 
cut,  which  are  supported  on  knife  edges.  The  short  balance 
lever  in  the  foreground  is  attached  to  the  rocker  arms  by  a  chain 
and  is  likewise  pivoted  on  knife  edges,  while  it  carries  on  its 
longer  sector,  suspended  by  a  chain,  the  adjustable  counter- 
balance weights.  The  current  can  be  adjusted  at  will  within 
reasonable  limits  merely  by  adding  or  taking  off  counter- 
balance weights.  The  whole  apparatus  is  enclosed  in  a  deeply 
fluted  cylindrical  cast  iron  case  filled  with  paraffin  oil,  which 
serves  the  double  purpose  of  giving  high  insulation  and  also 
damping  the  oscillations  of  the  floating  coils.  Hence  the  sys- 
tem has  been  jocularly  dubbed  the  "tub"  system,  and  the 
name  has  every  appearance  of  sticking.  Transformers  of  this 
type  are  built  for  as  large  a  load  as  100  series  arc  lights,  in 
which  instance  they  are  usually  arranged  on  the  multi-circuit 
plan,  with  two  5o-light  circuits.  Some  of  these  big  tubs 
have  four  primaries  and  four  secondaries,  while  the  smallest 
sizes  have  but  one  of  each. 

The  diagram,  Fig.  235,  gives  a  very  clear  idea  of  the  circuits 
of  this  simpler  form  of  the  apparatus,  and  of  the  shifting  of 
the  secondary  coil  under  changing  load.  The  secondary  is 
shifted  by  hand  into  the  short  circuit  position  and  a  plug 
short  circuiting  switch  inserted  just  prior  to  starting  up,  and 
when  the  primary  current  is  on  the  short  circuiting  switch 
is  withdrawn,  throwing  the  current  upon  the  lamps. 

These  transformers  regulate  promptly  and  steadily,  and  hold 
the  current  quite  accurately  constant  from  full  load  down  to 
as  low  as  'one-quarter  load.  Their  efficiency  as  transformers 
of  energy  is  high  in  spite  of  a  rather  unfavorable  form  of  the 
magnetic  circuit,  being  95-96  per  cent,  at  full  load  in  the 


PLATE   XVII. 


CENTRES  OF  DISTRIBUTION. 


average  sizes.  From  the  nature  of  the  case,  however,  the 
power  factor  of  such  apparatus  is  not  as  high  as  would  be 
desirable,  being  about  80  per  cent,  at  full  load,  and  falling 
off  in  practically  linear  proportion  as  the  load  decreases.  For 
this  reason  the  apparatus,  when  put  into  action,  throws  a  nasty 
inductive  load  upon  the  system,  and  it  is  good  policy  to  cut  it 
in  with  a  water  rheostat  in  the  primary  circuit  so  that  the 
load  may  go  on  gradually.  An  ordinary  barrel  nearly  filled 
with  pure  spring  water,  with  a  fixed  electrode  at  the  bottom 
and  a  movable  one  at  the  top,  each  a  little  smaller  than  the 
barrel  head,  makes  a  very  efficient  rheostat  for  an  ordinary 
circuit  of  2,000  volts  or  so. 

Owing  to  the  low  power  factor,  such  apparatus  should  not 
be  used  on  circuits  likely  to  be  worked  much  at  partial  load. 


FIG.  235. 

It  is  very  well  suited,  however,  to  street  lighting,  and  has  come 
into  very  extensive  use. 

A  similar  device  is  used  in  the  practice  of  the  Westinghouse 
Company,  differing,  however,  in  that  the  transformer  and 
regulator  functions  are  not  combined.  The  regulator  is 
shown  in  Fig.  2,  PI.  XVII,  and  in  virtue  of  what  has  already 
been  said  the  cut  is  self-explanatory.  The  balanced  floating 
coil  inserts  automatically  the  reactance  necessary  to  hold 
the  current  constant.  Regulators  are  made  for  a  range 
of  action  varying  from  25  per  cent,  to  100  per  cent,  of  the 
whole  load,  according  to  the  requirements  of  the  case,  and, 


514  ELECTRIC    TRANSMISSION  OF  POWER. 

of  course,  are  of  greater  size  and  cost  according  to  the 
range  required.  They  hold  the  current  closely  at  a  uniform 
value,  and  have,  probably,  at  full  load  a  somewhat  better 
power  factor  than  the  combined  apparatus  just  described. 
They  have  the  same  rapid  falling  off  of  power  factor  at  low 
loads,  however,  and  must  be  used  in  connection  with  a 
separate  static  transformer  to  give  the  required  voltage  on 
the  arc  circuit.  They  could,  of  course,  be  installed  directly 
on  the  distribution  circuit,  but  at  the  risk  of  a  ground  on 
the  arc  circuit  involving  the  whole  system  in  trouble,  so  that 
practically  they  are  regularly  used  with  transformers.  The 
efficiency  of  this  system  does  not  differ  materially  from  that 
of  the  tub  system  already  discussed,  and  the  operative  qualities 
of  both  are  much  the  same. 

The  series  alternating  arc  lights  thus  operated  have  come 
into  very  extensive  use  and  are  rapidly  driving  out  the  open 
continuous  current  arcs  for  street  lighting.  They  are,  of 
course,  always  enclosed,  and  give  a  very  steady  and  evenly 
distributed  light,  free  from  shadows  and  bright  zones,  which 
has  proved  highly  satisfactory  for  street  lighting. 

The  ordinary  series  alternating  arc  takes  about  6.5  amperes 
and  425  watts,  and  at  this  input  is  materially  better  as  an 
illuminant,  light  for  light,  than  the  so-called  1,200  c.  p.  open 
arc  or  the  enclosed  continuous  current  arc  taking  5  amperes- 
or  thereabouts.  To  compete  on  favorable  terms  with  the 
9.5  ampere  open  arcs,  or  the  6.5  ampere  enclosed  continuous 
current  arcs,  the  current  in  the  alternating  system  should  be 
carried  up  to  about  7.5  amperes,  at  which  point  it  is  fully  the 
equal  of  the  others  in  practical  street  lighting. 

Used  thus  on  the  street,  the  slight  noise  of  the  alternating 
arc  is  not  noticeable,  and  experience  has  shown  the  operation 
of  the  system  to  be  eminently  satisfactory.  For  commercial 
lights  the  series  alternating  arcs  are  not  to  be  commended, 
since  the  power  factor  of  the  regulating  devices  is  objection- 
ably low  at  partial  loads.  The  choice  for  such  work  lies  be- 
tween conversion  to  continuous  current  and  the  use  of  con- 
stant potential  alternating  arcs,  with  the  advantage,  at  the 
present  time,  rather  in  favor  of  the  former  expedient. 

The  general  principles  of  distribution  laid  down  hold  what- 
ever alternating  system  is  used.  Polyphase  and  other  modified 


CRN  TRES  OF  DIS  TRIE  U  TION. 


5*5 


alternating  systems  require  special  treatment  in  the  details  of 
distribution,  but  not  in  the  broad  methods  employed. 

Motor  service  should  generally  be  cultivated  as  a  desirable 
source   of  profit  and  an  excellent  way  of   raising  the  plant 


6AM 


M 

FIG.  236. 


6PM 


efficiency.  A  motor  load,  if  of  numerous  units  or  a  few 
steadily  loaded  ones,  is  remarkably  uniform.  Fig.  236  shows 
the  load  line  of  a  three-phase  power  transmission  plant. 
The  motor  load  consisted  of  about  fifty  induction  motors 
of  various  makes,  aggregating  nearly  350  rated  HP.  The 
curve  shows  the  primary  amperes  in  one  leg  of  the  circuit 
throughout  the  twenty-four  hours.  It  was  taken  on  a  day 
in  early  August  when  the  lamp  load  was  very  light  and 
reached  its  maximum  as  late  as  8  P.  M.  The  motor  load,  save 
for  the  sharp  decline  during  the  noon  hours,  was  very  steady, 
although  there  were  frequent  variations  through  a  range  of  a 
few  amperes,  too  brief  to  appear  on  the  diagram.  In  this 
case  and  at  this  season  of  the  year  there  is  no  "lap  load." 


516  ELECTRIC   TRANSMISSION  OF  POWER. 

The  distribution  is,  as  far  as  possible,  from  secondary  mains, 
and  even  in  winter,  when  the  lap  load  is  prominent,  although 
the  motors  still  require  the  major  part  of  the  output,  the  regu- 
lation of  the  system  is  admirable. 

Thus  even  a  heavy  motor  load  gives  very  little  trouble  with 
a  properly  designed  system  of  distribution  and  judicious  han- 
dling. The  things  to  be  feared  are  large  motors  running  on 
very  variable  load,  motors  with  bad  power  factors  carried  by 
overloaded  transformers,  and  overloaded  conductors  during 
the  period  of  lap  load.  Now  and  then  a  system  is  installed  for 
motor  service  only  or  with  special  motor  circuits.  In  this  case 
it  should  be  remembered  that  there  is^no  need  for  any  very 
close  uniformity  of  voltage  throughout  the  system,  and  that  to 
attempt  it  means  waste  of  time  and  money.  The  circuits  can 
be  laid  out  with  reference  to  the  desired  efficiency  alone,  for 
in  most  cases  even  10  per  cent,  variation  in  voltage  between 
one  motor  and  another  is  of  little  consequence. 

The  distribution  of  power  from  scattered  sub-stations  fed 
by  a  common  line  involves  some  of  the  most  intricate  and 
puzzling  problems  to  be  found  in  power  transmission.  Such 
distributions  generally  arise  from  an  attempt  to  supply  from  a 
common  power  plant  energy  for  divers  purposes  to  several 
separate  towns  or  regions,  having  different  requirements.  In 
the  main  such  plants  require  special  treatment  in  order  to 
secure  decent  .service.  A  great  variety  of  cases  may  arise, 
almost  every  plant  having  peculiarities  of  its  own,  but  in 
general  they  will  fall  into  one  of  the  three  following  categories: 

1.  Radial  distribution  from  a  centrally  located  station. 

2.  Radiating  distribution  from  an  eccentric  station. 

3.  Linear  distribution. 

i.  The  first-mentioned  class  consists  of  those  plants  which 
supply  from  a  single  station  power  to  different  localities  lying 
in  different  directions,  and  generally  at  different  distances. 
Fig.  237  shows  the  character  of  the  conditions  thus  met.  A  is 
a  generating  station,  the  position  of  which  may  be  determined 
by  various  reasons — the  existence  of  a  valuable  water  power 
being  the  commonest;  B,  C,  D,  E,  F,  are  the  various  points  to 
be  supplied  with  power.  They  may  be  at  any  distances,  and 
of  any  sizes  or  natures.  Usually  the  greatest  distance  involved 
will  not  be  coupled  with  the  greatest  load,  and  the  situation 


CENTRES  OF  DISTRIBUTION.  517 

is  otherwise  inconvenient.  If  all  the  loads  were  large,  the 
simplest  procedure  would  be  to  install  one  or  more  generators 
for  each  circuit  and  operate  them  independently.  Or  if  by 
good  luck  two  or  more  load  points  were  of  similar  size,  dis- 
tance, and  character,  they  would  naturally  be  operated  as  if 
they  were  one. 

To  consider  methods  of  operation  more  in  detail,  imagine  a 
system  consisting  of  the  station  A^  a  load  at  B  consisting  of 
150  KW  in  lights  and  motors,  largely  the  latter,  distant  3 
miles  ;  and  a  load  at  E,  6  miles  away,  of  250  KW,  mostly 
incandescent  lamps.  At  both  B  and  E,  it  would  be  desirable 
to  distribute  at  the  voltage  of  transmission  without  a  general 
reducing  station.  In  such  a  plant  it  might  be  possible  to 
operate  B  and  E  from  separate  generators,  compounding 
them  or  using  the  regulating  methods  already  described. 
But  if  day  lighting  at  E  is  to  be  attempted,  it  would  be  neces- 
sary either  to  run  one  dynamo  all  day  at  a  trivial  load  or  to 
throw  this  day  work  in  on  the  other  circuit  and  take  the  volt- 
age as  it  chanced  to  come. 

With  the  ordinary  amount  of  loss  in  the  line  A  By  the 
result  would  be  decidedly  bad  regulation  at  E\  with  only 


FIG.  237. 

motors  at  B  or  E  the  case  would  be  very  simple  ;  the 
station  would  be  regulated  with  reference  to  the  lighting 
load  alone,  but  with  lights  at  both  places  there  must  be  good 
regulation  at  both.  During  the  day  at  least  it  woulr'  be 
desirable  to  work  both  lines  from  the  same  generator.  The 
first  step  in  this  direction  would  be  to  install  at  A  a  hand 
regulator  to  control  the  line  A  E.  This  might  be  such  a 
device  as  that  described  on  p.  467,  or  even  a  choking  coil  of 
variable  reactance  and  sufficient  range  of  action  to  prevent  the 
generator,  in  compounding  or  otherwise  regulating  for  By  from 


518  ELECTRIC   TRANSMISSION  OF  POWER. 

causing  undue  variations  at  E.  As  already  pointed  out,  a 
motor  load  is  often  fairly  steady  except  at  certain  times,  so 
that  the  regulator  would  require  little  attention  save  in  the 
early  morning  and  at  noon.  Before  the  motor  load  fell  off  in 
the  afternoon  it  would  probably  be  desirable  to  start  a  sepa- 
rate dynamo  for  E. 

In  operating  both  lines  regularly  from  the  same  gener- 
ators, hand  regulation  on  at  least  one  of  them  would  become 
necessary;  on  which  one  is  a  matter  of  relative  convenience. 
If  the  distances  A  £,  A  E  were  much  smaller,  not  more  than 
two  or  three  miles,  it  might  be  feasible  to  install  both  lines  for 
small  and  equal  drop — not  over  2  per  cent. — so  that,  if  the 
dynamo  were  compounded  for  an  equal  amount,  the  possible 
variations  of  voltage  would  be  trifling.  Such  a  plan  cannot, 
however,  give  really  good  regulation  over  any  save  very  short 
distances  without  inordinate  expense  for  copper.  This  sort  of 
regulation  by  general  average  has  been  tried  too  often  already, 
with  disastrous  results,  and  is  quite  out  of  place  in  serious 
power  transmission  work. 

If  A  E  were  10  or  15  miles  long  the  manner  of  operation 
would  become  a  still  more  troublesome  question.  Raising 
and  reducing  transformers  would  generally  be  used,  and  the 
best  plan  would  probably  be  to  install  a  pressure  regulator  in 
connection  with  the  raising  bank  of  transformers,  and  let 
the  shorter  line  be  taken  care  of  by  the  compounding  of  the 
generator  or  by  a  pressure  regulator  of  its  own.  The  latter 
procedure  is  somewhat  preferable.  For  if  the  drop  in  the 
lines  be  5  or  10  per  cent,  and  the  loads  variable,  the  work 
of  regulation  will  be  lessened  by  compounding  the  generator, 
if  at  all,  for  constant  potential  at  its  own  terminals.  The 
range  of  the  hand  regulation  is  thus  lessened  since  there  is 
no  attempt  at  over  compounding;  and  two  regulators  requir- 
ing occasional  adjustment  are  easier  to  handle  than  one  which 
requires  continual  juggling  to  produce  indifferent  results. 

In  certain  cases  of  heavy  load  there  may  be  a  regular  sub- 
station at  B  or  at  E,  the  distribution  at  the  other  point  being 
direct.  Then  the  regulation  question  is  better  transferred  to 
the  sub-station,  the  generator  being  regulated  for  the  loss  in 
the  other  line  which,  as  its  load  will  usually  be  relatively 
small,  should  have  a  comparatively  small  drop. 


CENTRES  OF  DISTRIBUTION'.  519 

The  most  troublesome  case  that  can  arise  is  when  power  is 
to  be  furnished  to  a  street  railway  at  B  or  £,  in  addition  to 
a  general  lighting  and  motor  service.  A  railway  load  is  so 
violently  variable  that  it  cannot  be  operated  in  direct  connec- 
tion with  an  incandescent  service  unless  this  latter  with  the 
general  motor  load  is  so  great  as  to  quite  dwarf  the  variations 
of  railway  load.  Frequently,  therefore,  a  separate  generator 
should  be  devoted  to  the  railway  work.  In  case  this  cannot 
be  done  without  great  inconvenience,  it  may  become  neces- 
sary to  install  a  sub-station  at  which  the  lighting  circuits  can 
be  regulated  either  by  hand  or  automatically. 

Suppose  now  that  the  problem  is  complicated  by  the  addi- 
tion of  loads  at  C,  D,  and  F.  These  lines  will  be  treated  on 
the  same  general  principles  as  the  first  two.  To  begin  with, 
any  line  operating  motors  alone  can  be  worked  direct  from 
the  generator.  Even  if  all  the  loads  be  mixed  in  character, 
two  or  more  can  often  be  found  which  through  similarity  of 
conditions  can  be  worked  together  in  parallel,  "either  by  a 
common  regulator  or  by  compounding  the  generator.  The 
others  should  be  treated  as  already  indicated.  At  the  worst 
it  might  be  necessary  to  install  a  regulator  for  each  line. 
This  is  not  really  so  burdensome  as  might  be  supposed, 
since  several  of  the  regulators  will  usually  require  infrequent 
attention,  so  that  one  man  can  manipulate  the  whole  set. 
This  line  of  action  is  similar  to  that  followed  in  most  large 


FIG.  238. 

central  stations,  where  feeder  regulation,  although  rather  a 
nuisance,  is  successfully  accomplished  without  any  particular 
difficulty.  Feeder  regulators  for  alternating  circuits  have, 
however,  by  no  means  received  the  attention  that  is  their  due. 
Pressure  wires  from  each  load  point  are  desirable,  though, 
if  the  load  is  such  that  the  inductive  drop  is  small  or  quite 
steady,  the  regulator  can  be  easily  adjusted  in  accordance 
with  the  current  on  the  line. 


520  ELECTRIC   TRANSMISSION  OF  POWER. 

In  the  transmission  and  distribution  of  power  from  an  ec- 
centric station,  the  difficulties  are  many  unless  recourse  be 
taken  to  a  regular  sub-station.  Fig.  238  shows  a  typical  situa- 
tion. Here  A  is  the  generating  station  and  B,  C,  Z>,  £,  J?t 
are  the  load  points.  If  the  distance  from  A  to  the  nearest 
load  is  great  enough  to  require  raising  and  reducing  trans- 
formers, it  is  generally  best  to  install  a  reducing  sub-station 
.worked  like  the  central  station  A,  Fig.  237.  Sometimes, 
however,  it  is  only  half  a  dozen  miles  or  so  from  A  to  the 
group  of  load  points.  The  case  is  similar  to  that  discussed 
in  the  first  part  of  this  chapter,  save  that  the  load  is  in  several 
distinct  localities  instead  of  being  generally  distributed. 
From  this  difference  the  complication  arises.  A  certain  pro- 
portion of  cases  can  be  treated  readily,  however,  by  choosing 
a  point  G  near  the  centre  of  load  and  then  running  the  lines 
G  B,  G  C,  G  D,  G  E,  G  F  with  i  or  2  per  cent,  loss  wherever 
lights  are  to  be  furnished.  Then  by  holding  the  voltage  con- 
stant at  G  or  slightly  over  compounding  at  that  point,  suffi- 
ciently good  service  can  often  be  given. 

If  the  loads  are  very  unequal  in  size,  G  may  be  chosen  at  or 
near  the  most  important  point  and  lines  run  to  the  others  as 
before,  with  the  regulation  question  confined  practically  to 
the  first.  If  the  load  points  are  quite  numerous  and  scat- 
tered, Fi&  239  may  be  a  preferable  plan.  Here  two  lines 


FIG.  239- 

A  B  and  A  C  are  run  and  a  group  of  load  points  is  served 
from  the  terminal  of  each  line.  The  groups  shown  are  about 
equal,  but  sometimes  it  would  be  desirable  to  run  a  separate 
line  for  a  single  point  where  the  load  was  peculiarly  heavy  or 
troublesome. 


CENTRES  OF  DISTRIBUTION.  521 

These  scattered  distributions  are  fortunately  mostly  for 
motor  service,  so  that  regulation,  in  practice,  is  often  easier 
than  the  situation  indicates.  They  sometimes  run  naturally 
into  the  linear  distribution,  which,  unless  of  trivial  size,  is  a 
thorn  in  the  flesh  of  the  engineer. 

Fig.  240  is  a  type  of  this  linear  distribution,  which  is  often 
met  with  in  large  transmission  work  and  especially  in  long 
distance  cases. 

The  power  station  A  is  mainly  intended  to  supply  lights  and 
power  at  B,  which  may  generally  be  supposed  to  be  the 
largest  town  in  the  immediate  region.  Incidentally  it  is  highly 
desirable  to  supply  lights  and  power  to  C,  D,  E,  F,  G,  towns  or 
manufacturing  points  at  which  electric  power  is  needed.  The 
main  line  A  B  may  be  taken  as  20  miles,  which  is  enough 
to  disclose  most  of  the  difficulties. 

Of  course  the  line  must  be  operated  at  high  voltage  with 
raising  and  reducing  transformers.  In  nearly  every  case  the 


FIG.  240. 

latter  would  be  placed  in  a  regular  sub-station,  with  appro- 
priate regulating  apparatus  for  keeping  uniform  voltage 
throughout  the  primary  and  secondary  networks  in  B.  The 
loss  of  voltage  in  the  line  above  may  be  assumed  at  10  per 
cent,  and  the  primary  pressure  at  B  as  15,000  volts.  As  B 
comprises  by  far  the  largest  and  most  important  part  of  the 
load,  attention  should  be  first  directed  to  complete  regulation 
at  that  point. 

This  can  be  best  attained  by  first  holding  the  primary  pres- 
sure at  B  constant  by  compounding  or  other  regulation  at  A, 
and  second, by  careful  regulation  of  the  primary  and  secondary 
feeders  in  the  sub-station.  In  fact  the  whole  transmission 
must  first  be  treated  with  respect  to  results  at  B,  while  never- 
theless it  is  necessary  to  scatter  power  along  the  line  at  the 
points  indicated.  There  may  be  present  all  sorts  of  require- 


52*  ELECTRIC   TRANSMISSION  OF  POWER. 

ments.  For  example,  at  C  there  may  be  required  1,000  in- 
candescent lamps  and  a  few  motors;  at  D  500  incandescents; 
at  E,  a  50  HP  motor  and  300  incandescents,  at  ^300  incan- 
descents, and  at  G  200  HP  in  motors  and  200  lamps. 

Frequently  the  load  at  one  or  more  points  may  consist  of 
motors  only.  This  case  is  not  included  above,  since  no 
special  regulation  is  needed;  the  power  has  only  to  be  trans- 
formed from  the  line  voltage  to  that  of  the  motors,  neglecting 
the  effect  of  varying  loss  in  the  line. 

Each  of  the  cases  noted  involves  the  question  of  regulation 
in  a  somewhat  troublesome  form;  at  Z>,  for  example,  the  con- 
ditions under  which  incandescent  lamps  must  be  supplied  are 
most  severe.  To  begin  with,  at  the  nearest  point  of  the 
main  line  A  B,  the  voltage  may  change  by  about  6  per  cent., 
owing  to  varying  loss  in  the  line;  the  branch  to  D  causes  a 
trifle  more  variation,  the  drop  in  the  transformers  still  more, 
and  finally  there  must  be  added  the  loss  in  the  secondaries  up 
to  the  lamps.  In  all,  these  cumulative  variations  in  voltage 
may  be  10  per  cent,  or  more.  At  best,  this  means  5  per  cent, 
change  of  voltage  above  and  below  the  normal.  This  is  too 
great  to  allow  what  can  be  called  good  service,  although  worse 
is  sometimes  given.  In  fact  such  variation  ought  to  be 
classified  as  outrageously  bad.  To  better  matters,  two 
methods  are  available. 

First,  one  may  use  a  hand  regulator  in  connection  with 
the  reducing  transformers;  for,  in  so  large  a  system  as 
that  involved,  the  changes  in  voltage  are  relatively  slow, 
and  the  conditions  of  load  may  be  such  that  over  com- 
pounding on  the  main  line  may  partially  compensate  for 
the  losses  elsewhere.  Or  second,  the  lights  may  be  oper- 
ated by  a  dynamo  driven  by  a  synchronous  motor.  This 
procedure  adds  somewhat  to  the  expense  and  trouble  but 
completely  eliminates  the  loss  in  the  line,  since  the  speed  of 
the  motor  is  independent  of  the  applied  voltage,  and  incident- 
ally, of  the  load. 

For  small  outputs  a  good  induction  motor  serves  the 
purpose  well,  for  it  is  simpler  to  operate  than  the  syn- 
chronous variety  and  can  be  made  remarkably  insensitive 
to  changes  of  load  and  voltage.  This  motor  generator 
device  is  an  admirable  resource  when  a  very  variable  line 


CENTRES  OF  DISTRIBUTION.  523 

voltage  must  be  dealt  with.  In  making  the  installation  for 
a  point  like  D  the  actual  variation  of  the  pressure  at  the  point 
of  tapping  the  main  line  should  be  ascertained,  and  the  effect 
of  the  subsequent  losses  up  to  the  lamps  should  be  computed. 
If  the  resultant  changes  are  frequent  and  considerable,  a 
motor  generator  gives  the  best  result.  For  gradual  and 
moderate  changes  an  occasional  touch  at  a  regulator  may  be 
all  that  is  needed,  and  now  and  then  the  resultant  variation 
will  prove  to  be  not  more  than  2  per  cent,  above  or  below  an 
assumed  normal  for  the  lamps,  in  which  case  the  regulation 
often  may  take  care  of  itself. 

At  C  there  is  a  distribution  equivalent  to  that  from  a  small 
central  station.  The  line  pressure  will  generally  have  to  be 
twice  reduced  before  feeding  the  lamps.  The  choice  of 
methods  is  the  same  as  in  the  case  just  discussed,  except  that, 
with  the  losses  of  a  double  transformation  and  rather  scat- 
tered service,  regulation  cannot  be  left  to  chance.  Generally 
in  a  station  of  this  size  some  regulation  due  to  the  distribution 
itself  will  have  to  be  provided  for,  and  the  simplest  course  is 
to  establish  a  sub-station  with  one  or  more  pressure  regulators. 
This  is  operated  just  like  the  sub-station  at  B,  being  merely  on 
•a  much  smaller  scale,  seldom  large  enough  to  require  the  use 
of  pressure  wires.  A  careful  study  of  local  conditions,  how- 
ever, is  needful  to  enable  one  to  discriminate  between  the  two 
methods  mentioned. 

At  the  station  E  the  motor  will  take  care  of  itself,  but  the 
lamps  might  give  trouble  owing  to  variations  in  motor  load. 
If  these  are  great  and  sudden,  nothing  save  running  from  the 
motor  a  generator  for  the  lights  will  answer,  and  even  that  will 
not  be  entirely  satisfactory.  If  the  load  is  steady  and  the  lights 
regularly  in  use,  as  would  be  common  in  factory  service,  the 
loss  in  the  branch  line  to  E  and  the  secondaries  can  be  ad- 
justed so  that  if  the  voltage  at  B  is  kept  constant  by  regula- 
tion at  A,  that  at  E  will  be  nearly  so.  This  device  is  probably 
the  one  best  suited  to  give  good  service  at  F.  For  G  the 
same  method  holds,  but  with  so  large  a  proportion  of  motor 
load,  separate  transformers  for  the  lights  are  almost  necessary. 
In  cases  where  there  is  no  regulation  at  A  for  the  loss  in  the 
line,  pressure  regulators  or  sometimes  motor  generators  will 
have  to  be  used  at  E,  F,  G. 


524  ELECTRIC   TRANSMISSION  OF  POWER. 

The  various  cases  of  linear  distribution  just  considered  are 
of  necessity  treated  little  in  detail,  since  they  are  so  much 
modified  in  practice  by  special  circumstances.  Enough  has 
been  said,  however,  to  indicate  the  methods  to  be  followed  and 
to  show  how  tactfully  this  class  of  problems  must  be  treated. 

Finally  comes  that  very  important  class  of  cases  which 
involves  the  distribution  of  transmitted  energy  from  large 
reducing  stations.  Such  is  the  normal  condition  of  affairs 
whenever  power  is  transmitted  to  a  city  in  large  amounts  for 
lighting  and  motor  service.  Passing  over  a  few  instances  in 
which  this  power  may  be  mainly  utilized  for  driving  by  motors, 
or  replacing  by  rotary  transformers  existing  central  stations, 
one  is  contronted  by  the  problem  of  constituting  a  great 
distributing  system  for  alternating  currents;  a  system  gen- 
eral enough  to  be  available  for  every  service,  and  perfect 
enough  to  compare  favorably  with  the  great  networks  now 
worked  by  continuous  currents.  Until  very  recently  this 
problem  would  have  been  insoluble  in  any  practicable  way, 
but  to-day,  thanks  to  the  modern  alternating  systems  and 
to  the  intelligent  use  and  arrangement  of  large  transformer 
units,  it  is  possible  substantially  to  duplicate  -in  conven- 
ience and  efficiency  the  best  direct  current  systems,  while 
retaining  the  enormously  valuable  advantage  of  using  high 
tension  feeders.  It  must  not  be  supposed,  however,  that 
the  same  procedure  must  suit  both  cases — the  results  but 
not  necessarily  the  methods  must  be  in  full  accord. 

The  basis  of  each  system  must  be  a  carefully  laid  out  network 
of  working  conductors,  giving  throughout  the  area  of  service 
a  substantially  uniform  voltage  as  high  as  can  conveniently  be 
employed  in  the  various  consumption  apparatus — lights,  motors, 
and  so  forth.  This  voltage  is  practically  determined  by  that 
of  the  incandescent  lamps  which  are  available.  A  few  years 
ago  100  to  no  volts  was  the  working  limit  of  effective  voltage 
between  incandescent  service  wires  (not  of  course  the  extreme 
voltage  to  be  found  between  any  two  wires  of  the  system).  Of 
late  the  majority  of  important  stations  employ  lamps  of  115- 
120  volts.  Now  and  then  120-130  volts  is  reached,  and  very 
recently  there  has  been  a  strong  movement  toward  boldly 
doubling  the  usual  voltages  and  employing  lamps  made  for 
200-250  volts. 


CENTRES  OF  DISTRIBUTION.  525 

A  considerable  number  of  scattered  small  plants  use  such 
lamps,  and  in  a  few  cases  central  stations  have  adopted  them 
in  connection  with  three-wire  systems,  using  thus  about  440 
to  500  volts  between  the  outside  wires.  There  is  a  decided 
tendency  in  this  direction,  and  occasional  stations  have  under- 
taken to  change  to  this  double  voltage,  at  least  to  the  extent  of 
trying  220  volt  lamps  extensively.  At  present  these  lamps  are 
of  somewhat  uncertain  quality  and  rather  high  price,  but  they 
have  been  rapidly  improved,  both  here  and  abroad. 

It  is  undoubtedly  much  harder  to  get  an  efficient  and  durable 
filament  for  220  than  for  no  volts  at  a  given  candle  power. 
Such  a  filament  is  necessarily  very  slender  and  correspond- 
ingly fragile.  If  two  no  volt  filaments  mounted  in  series 
would  answer,  the  task  would  be  simple,  but  such  a  combi- 
nation gives  double  the  required  candle  power,  which  is 
generally  undesirable.  The  net  result  of  present  experience 
is  that  while  220  volt  lamps  can  be  made  to  give  excellent 
results  in  efficiency  and  life  they  are,  as  a  rule,  both  poorer 
and  costlier  than  the  corresponding  lamps  of  half  the  voltage. 
From  the  nature  of  lamp  manufacture  this  condition  is  likely 
to  remain,  in  perhaps  lessened  degree,  even  when  the  pro- 
duction of  these  high  voltage  lamps  is  extensive.  The  ques- 
tion between  the  two  from  a  commercial  standpoint  will 
ultimately  be  a  close  one,  although  at  present  the  advantage 
is  altogether  on  the  side  of  the  lower  voltage  in  most  instances. 
The  high  voltage  lamps  are  most  satisfactory  when  of  20  to  32 
candle  power  and  worked  at  3.5  to  4  watts  per  candle.  Under 
such  conditions  the  filaments,  being  somewhat  thicker  than  in 
a  16.  c.  p.  lamp  of  similar  voltage,  and  being  worked  at  a  lower 
temperature  than  the  high  efficiency  lamps,  give  a  reasonably 
good  life. 

Until  much  experience  has  been  accumulated  with  reference 
to  the  high  voltage  lamps,  their  use  in  any  considerable  under- 
taking cannot  safely  be  recommended.  It  would  be  par- 
ticularly unwise  to  attempt  it  in  a  large  transmission  plant, 
where  any  trouble  with  the  lamps  would  inevitably  be  charged 
against  the  general  system.  It  is  better,  then,  to  select  for 
incandescent  lighting  a  voltage  only  so  high  as  has  been 
thoroughly  tried — say  115  to  120. 

The   resulting  service  voltage  on   the   secondary  network 


526  ELECTRIC   TRANSMISSION  OF  POWER. 

depends  on  the  system  of  distribution  employed.  There  are 
actually  employed  for  primary  or  secondary  distribution  with 
alternating  currents  about  a  round  dozen  of  distinct  methods, 
more  or  less  convenient  and  inconvenient,  and  requiring  very 
various  amounts  of  copper  for  distributing  the  same  amount 
of  energy  at  the  same  loss  and  distance.  Several  of  them  are 
very  convenient  and  valuable,  others  have  as  their  only  excuse 
for  existence  the  desire  to  exploit  a  novelty  or  to  evade  some- 
body's patent. 

The  simplest  of  them  all  is  the  ordinary  two-wire  system 
worked  with  alternating  currents.  In  this  the  maximum  volt- 
age of  the  lamps  is  the  maximum  voltage  of  the  secondary  sys- 
tem. To  avoid  this  limitation  and  to  secure  the  ability  to  run 
motors  is  the  principal  function  of  the  various  modifications, 
polyphase  and  other,  which  make  up  the  remainder.  As  these 
various  systems  are  often  exploited  it  is  worth  the  while  to 
review  them  briefly,  with  special  reference  to  economy  of  cop- 
per and  convenience  of  installation  on  a  large  scale  for  the 
purpose  we  are  considering.  The  two-wire  system  is  shown 
diagrammatically  in  Fig.  241.  Its  main  advantage  is  extreme 


FIG.  241. 

simplicity.  It  requires  the  same  amount  of  copper  as  a  two- 
wire  direct  current  system  at  the  same  effective  voltage,  and 
is  installed  in  the  same  general  way,  except  that,  owing  to 
the  peculiarities  of  alternating  currents  already  explained, 
very  large  single  wires  are  undesirable  and  armored  conduits 
for  underground  service  must  be  used  with  great  caution. 

As  to  motors  for  such  a  system  the  case  is  not  altogether 
what  one  would  desire.  Alternating  monophase  motors 
are  not  yet  so  satisfactory  for  general  service  as  those  of 
some  other  types,  more  particularly  as  regards  starting  and 
severe  service,  and,  until  considerable  improvement  is  made 
in  them,  the  pure  monophase  system  is  severely  handicapped. 
The  two-wire  arrangement  is  always  at  rather  a  disadvantage 
in  the  amount  of  copper  required  both  for  feeders  and  service 
mains. 


CENTRES  OF  DISTRIBUTION.  527 

The  most  obvious  modification  of  this  distribution  is  its 
evolution  into  a  three-wire  system  such  as  is  familiar  in 
Edison  stations.  The  extreme  working  voltage  is  at  once 
doubled,  and  thus  with  the  same  voltage  at  the  lamps,  the  cost 
of  copper  is  greatly  reduced.  If  the  copper  for  a  given  two-wire 
system  be  taken  as  100  that  for  the  corresponding  three-wire 
system  is  31.25,  assuming  that  the  so-called  neutral  wire  is  of 
one-half  the  cross  section  of  either  of  the  others.  Fig.  242 
shows  this  familiar  arrangement  in  diagram.  Like  every 
other  system  which  saves  copper,  a  three-wire  distribution  is 
subject  to  certain  inconveniences.  In  the  first  place,  it  is  nec- 
essary to  carry  three  wires  instead  of  two  over  substantially 
the  whole  working  area.  Secondly,  the  lamps  must  be  nearly 
equally  divided  between  the  two  sides  of  the  system.  This 
balancing  of  the  load  is  not  particularly  troublesome  in  a  well- 
managed  plant,  and  general  experience  has  shown  that  the 
gain  in  copper  far  outweighs  this  disadvantage. 

This  three-wire  distribution  has  been  largely  used  for  alter- 
nating current  work.  It  is  sometimes  very  convenient  when 
applied  to  single  or  grouped  transformers  for  the  lighting  of 


FIG.  242. 

large  buildings  and  regions  in  which  balance  of  load  is  easily 
preserved.  In  such  case  the  transformers  are  supplied  from 
high  voltage  feeders,  generally  arranged  on  the  two-wire  system. 
As  a  rule,  however,  proper  balancing  is  not  easy  in  iso- 
lated districts  and  the  best  use  of  the  three-wire  system  is  for 
a  general  network  of  secondary  mains,  the  voltage  upon  which 
can  be  controlled  from  a  central  station.  In  an  ordinary 
direct  current  plant  the  feeders  are  of  course  at  low  voltage, 
and  a  great  advantage  is  gained  for  the  alternating  arrange- 
ment by  feeders  at  two  or  more  thousand  volts  supplying  the 
mains  through  transformers.  As  regards  motors,  the  alter- 
nating current  three-wire  system  is  on  substantially  the  same 
basis  as  the  alternating  two-wire  system. 

More  complicated  pure  monophase  systems  are  seldom  used, 


528  ELECTRIC   TRANSMISSION  OF  POWER. 

although  there  is  one  instance  at  Portland,  Ore.,  of  a  four- 
wire  feeder  system;  derived,  however,  from  polyphase  gener- 
ators. Fig.  243  shows  the  arrangement  of  the  lines,  which  are 


I        * 

I 

FIG.  243. 

operated  in  general  like  a  three  wire-plant  and  require  similar 
care  in  balancing,  with  the  additional  complication  of  running 
four  wires  and  balancing  three  branches.  The  saving  in  cop- 
per is  of  course  very  great,  the  amount  needed,  allowing  half 
the  area  of  the  outside  wires  for  each  neutral,  being  about 
16.6  against  100  for  the  two-wire  plant.  The  corresponding 
five-wire  system  may  be  passed  over,  as  it  is  not  used  at  all  for 
alternating  currents,  nor  extensively  in  any  way. 

Next  in  proper  order  comes  the  so-called  monocyclic  system,, 
which  is  essentially  a  monophase  system,  but  heterophase  with 
reference  to  the  operation  of  motors.  Its  principal  features 
have  already  been  explained.  So  far  as  lights  are  concerned, 
it  is  simply  the  monophase  system  already  described  in  both 
the  two-wire  and  three-wire  forms.  The  "  power  wire,"  which 
supplies  magnetizing  current  for  the  fields  of  the  motors,  is 
only  used  in  so  far  as  is  necessary  for  its  special  purpose,  and 
may  or  may  not  form  part  of  the  regular  network.  The  two- 
wire  monocyclic  system  shown  in  Fig.  244  describes  itself. 


AA/* 


FIG.  244. 

The  expense  and  trouble  of  installing  the  "  power  wire  "  is 
the  price  paid  for  the  ability  to  run  motors.  The  total  amount 
of  copper  is,  of  course,  governed  by  the  size  and  extent  of  the 
power  wire.  The  main  wires  must  accommodate  the  full 
current  of  the  generator,  for  motors  and  lights  must  often  be 
operated  together,  and  at  all  events  the  machine  must  be  fully 


CENTRES  OF  DISTRIBUTION.  5*9 

utilized.  The  power  wire,  on  the  other  hand,  has  to  carry  only 
a  part  of  the  current  used  in  the  motors.  In  a  system  heavily 
loaded  with  motors,  the  power  wire  might  be  one-half  the 
•cross  section  of  each  of  the  main  wires.  If  then  it  extended 
over  the  entire  system,  it  would  add  25  per  cent,  to  the  copper 
required  for  the  main  circuit.  Generally  its  size  or  extent 
would  be  less  than  that  just  noted.  The  total  copper  required 
for  a  monocyclic  system  is  then  variable.  Its  relative  amount 
may  vary  from  100,  when  the  system  is  operating  lights  alone, 
to  125  for  rather  extreme  cases  of  motor  load. 

The  same  general  properties  hold  good  for  the  three-wire 
monocyclic  system  shown  in  Fig.  245.  It  is  treated  like  any  other 
three-wire  system,  except  for  the  addition  of  the  power  wire 
wherever  required.  There  is  evidently  a  great  saving  of  copper 
over  the  two-wire  monocyclic,  secured  at  the  cost  of  running 
an  extra  wire  as  a  neutral  and  balancing  the  load  on  the  two 
branches.  The  relative  weight  of  copper  varies  from  31.25 
for  lights  only  to  say  40  when  the  motor  system  is  extensive. 


FIG.  345. 

Either  form  of  the  system  is  singularly  easy  to  install  and 
operate  in  plants  already  having  a  considerable  network  of 
lines,  since  there  need  be  no  rearrangement  or  balancing  of 
circuits,  but  only  an  additional  line  wire  running  to  the 
motors  installed  and  extended  hand  in  hand  with  the  motor 
service.  The  monocyclic  system  is  very  little  used  in  practice, 
however,  since  it  possesses  no  important  advantages  over 
ordinary  polyphase  systems  and  is  decidedly  less  satisfactory 
for  motor  service. 

Passing  now  to  the  polyphase  systems,  it  is  well  to  reiterate 
what  has  already  been  stated  in  explaining  them,  viz.,  that 
they  all  involve  about  the  same  principles  and  lead  dynamically 
to  about  the  same  results.  They  do,  however,  differ  consider- 
ably in  their  characteristics  as  applied  to  a  general  system  of 
distribution,  and  in  rather  interesting  ways. 

The  diphase  system  can  be  worked  either  with  four  wires, 


5  30  ELEC  TRIG   TRA  N  SMI  SSI  ON   OF  PO  WER. 

i.  <f.,  a  complete  and  independent  circuit  for  each  phase,  or 
with  three  wires.  The  former  arrangement  is  the  one  almost 
invariably  used.  The  two  circuits  can  be  worked  independ- 
ently for  lights,  but  must  be  united  to  allow  the  operation  of 
diphase  motors.  For  the  former  purpose  the  two  windings  of 
the  generator  may  be  treated,  save  in  one  important  respect, 
like  separate  monophase  alternators.  For  the  latter  purpose 
they  must  work  conjointly.  Fig.  246  shows  the  relations  of  the 


FIG.  240 

two  circuits.  In  a  general  system  it  is  the  best  plan  to  carry 
the  two  circuits  throughout  the  territory  to  be  covered.  In 
this  way  motors  can  be  run  anywhere.  Otherwise,  if  the  main 
circuits  covered  different  districts,  connecting  lines  might 
have  to  be  run  at  considerable  expense  for  copper  and  labor, 
uniting  the  two  systems.  Further,  when  the  two  circuits  are 
together,  it  is  easier  to  divide  the  load  evenly  between  them; 
which  is  desirable  to  prevent  one  circuit  of  the  generator  being 
overloaded  before  the  other  is  fully  used.  Incidentally  hand 
regulation  must  sometimes  be  used  for  one  or  both  circuits, 
unless  the  loads  are  equal  as  regards  drop  in  the  lines.  If  the 
generator  is  to  be  compound  wound,  the  two  phases  must 
be  equally  loaded  in  order  that  the  compounding  may  be  able 
to  hold  the  voltage  on  both  phases  alike.  It  must  not  be 
understood  that  unequal  loads  affect  the  voltage  as  in  a  three- 
wire  system — they  merely  produce  different  "drops,"  in  the 
two  systems,  which  cannot  be  equalized  by  the  generator. 

As  to  the  relative  amount  of  copper  required,  it  is,  when 
both  phases  are  run  together,  TOO.  If  separated,  this  may  be 
slightly  increased  by  cross  connections  for  motors. 

A  diphase  system  can  be  organized  with  each  phase  form- 
ing a  three-wire  system  like  Fig.  242.  This  doubles  the  work- 
ing voltage  and  so  saves  copper,  but  at  the  cost  of  very  serious 
complication.  The  full  distribution  requires  six  wires,  three 
per  phase,  and  these  must  be  carried  together  or  cross-con- 


CENTRES  OF  DISTRIBUTION.  S31 

nected  for  motors,  if  separated.  The  first  procedure — running 
two  three-wire  systems  side  by  side  over  the  same  district — 
involves  frightfully  complicated  wiring;  and  the  second,  if  the 
motors  are  at  all  numerous,  requires  a  troublesome  system  of 
subsidiary  lines.  In  either  case,  not  only  would  each  three- 
wire  system  have  to  be  balanced  in  itself,  but  the  two  must  be 
mutually  balanced  unless  hand  regulation  is  resorted  to  for 
one  or  both.  Altogether  the  diphase  system  with  separated 
phases  does  not  lend  itself  readily  to  distribution  for  lights 
and  motors  on  a  large  scale,  save  in  changing  over  existing 
monophase  systems,  for  which  it  happens  to  be  exceedingly  well 
suited.  Its  worst  features  are  the  large  amount  of  copper 
required  for  secondary  mains,  and  the  forbidding  complication 
of  any  attempt  to  secure  economy  by  using  the  three-wire 
distribution.  Like  the  diphase  inter-connected  system  about 
to  be  described,  and  certain  forms  of  the  three-phase  system, 
it  is  most  practicable  in  plants  of  moderate  size  not  requiring 
a  complete  sub-station  with  a  full  system  of  secondary  mains. 
The  interconnected  diphase  system,  Fig.  247,  employs  a 
common  return  for  the  two  phases.  It  has  been  often  pro- 
posed but  seldom  used,  for  a  good  practical  reason.  The  com- 
bined phases  are  unsymmetrical  with  respect  to  the  inductance 
of  the  system,  so  that,  even  when  the  two  sides  of  the  system 
are  equally  loaded,  the  voltages  between  the  common  wire  and 
the  mains  are  unequal  by  an  amount  proportional  to  the  induc- 


FIG.  247. 

tive  loss  in  the  lines.  Hence  it  is  unsuited  for  long  lines 
either  primary  or  secondary,  overhead  or  underground. 
The  lamps  on  the  two  sides  of  the  circuit  are  at  nearly 
the  same  voltage,  but  the  voltage  between  the  mains  is  so 
compounded  of  the  two  phases  as  to  give  increased  working 
pressure  enough  to  reduce  the  relative  amount  of  copper 
to  72.8  under  the  most  favorable  circumstances.  The  system 
need  scarcely  be  considered  further,  since  it  is  more  curious 


53 2  ELECTRIC   TRANSMISSION  OF  POWER. 

than  valuable,  and  is  unlikely  to  be  employed  in  large  sub- 
station work. 

Three-phase  circuits  are  variously  arranged,  as  has  been 
already  indicated.  The  phases  are  very  seldom  separated,  for 
a  six-wire  circuit  is  too  complicated  for  general  use,  but  are 
usually  interconnected.  The  commonest  and  simplest  connec- 
tion is  shov/n  in  Fig.  248.  This  consists  of  only  three  wires, 
each  running  from  the  terminal  of  a  phase  winding  on  the 
armature.  Motors  are  connected  to  all  three  wires,  and 
lamps  between  any  two  wires.  The  voltage  is  the  same 
between  each  pair  of  wires,  provided  each  pair  be  equally 
loaded.  The  relative  amount  of  copper  required  is  75,  as 
explained  elsewhere.  Here,  as  always,  the  uniting  of  circuits 
to  save  copper  is  accompanied  by  the  need  for  balancing  the 
loads.  Not  only  does  change  of  load  on  one  branch  change 
the  drop  in  the  other  two,  but  interacts  with  them  in  the 
transformers  and  generator.  The  disturbance,  however,  is 
fortunately  trivial  in  amount,  except  for  very  great  inequali- 


u  u 

FIG.  248. 

ties  of  load  or  for  abnormally  large  line  loss.  With  ordinary 
losses  in  the  line  it  is  absolutely  negligible  when  the  circuits 
at  full  load  are  balanced  within  10  or  15  per  cent.,  and  at  light 
loads  far  greater  inequality  will  have  no  perceptible  effect. 
With  ordinary  care  in  arranging  the  installation  the  question  of 
balance  never  assumes  any  considerable  importance,  and  need 
not  do  so  even  when  very  close  regulation  is  desired.  The 
main  objection  to  the  system  of  Fig.  248  is  the  considerable 
amount  of  copper  required  for  a  distribution  by  secondary 
mains  as  compared  with  the  ordinary  three-wire  systems.  Its 
salient  advantage  is  its  ability  to  handle  motors  and  lights  with 
equal  facility  on  a  system  composed  of  only  three  wires,  and  with 
some  saving  of  copper.  The  trouble  of  approximately  balanc- 
ing the  three  branches  is  regarded  as  insignificant  by  those 
who  are  operating  such  systems.  This  three-phase  distribu- 


CENTRES  OF  DISTRIBUTION.  533 

tion  is  sometimes  taken  from  the  three  common  junctions  of 
a  mesh  connection,  but,  while  for  motors  the  connection  is  a 
matter  of  indifference,  the  star  is  to  be  preferred  in  a  mixed 
service. 

A  far   better  system   for   sub-station   distribution   is   that 
shown  in  Fig.  249.     It  is  a  three-phase  system  with  a  neutral 


FIG.  249. 

wire  connected  to  the  neutral  point  of  the  three-phase  wind- 
ings. The  lamps  are  connected  between  this  neutral  wire  and 
the  several  main  lines.  The  result  is  that  the  working  volt- 
age of  the  lamps  is  the  voltage  from  either  line  to  the  neu- 
tral point,  while  the  working  voltage  of  the  system  is  1.73 
times  greater,  being  the  voltage  between  line  and  line.  Hence 
there  is  a  great  reduction  in  the  amount  of  copper  required, 
the  relative  weight,  as  compared  with  the  two-wire  monophase 
system,  being  only  29.2  if  the  neutral  wire  is  taken  of  cross 
section  equal  to  one-half  that  of  either  of  the  other  wires. 
This  system  must  be  balanced  approximately,  but  requires  less 
care  in  this  respect  than  the  ordinary  three-phase  connection 
just  described.  It  is  on  the  whole  better  adapted  for  large  dis- 
tributions of  mixed  lighting  and  power  than  any  other  of  the 
modern  alternating  systems,  since  it  combines  a  fairly  simple 
arrangement  of  wiring  with  very  great  economy  of  copper. 
It  lends  itself  readily  even  to  underground  service,  giving  a 
rather  simple  cable  construction  and  facilitating  testing.  It 
is  used  with  excellent  results  in  the  Folsom-Sacramento,  the 
Fresno,  and  other  important  transmission  plants,  for  the  main 
work  of  distribution. 

An  interesting  modification  of  the  three-phase  system  is 
that  used  in  the  city  of  Dresden  and  shown  in  Fig.  250. 
Here  the  system  is  constituted  in  the  ordinary  way,  but  two 
of  the  leads,  a  and  b,  are  arranged  to  carry  all  the  lighting, 
while  the  third  wire  c,  which  may  be  of  much  less  area,  is  used 
only  in  connection  with  the  motors.  It  may  even  sometimes  be 


534 


ELECTRIC   TRANSMISSION  OF  POWER. 


advantageous  to  increase  the  cross  section  of  two  of  the  arma- 
ture windings  at  the  expense  of  the  third.  A  machine  so  con- 
stituted would  have  fully  as  great  capacity  as  a  monophase 
machine  of  the  same  dimensions,  and  still  would  be  amply  able 
to  carry  any  ordinary  motor  loads.  Even  with  the  ordinary 
three-phase  winding  this  connection  may  be  used  without 
serious  reduction  of  output  as  compared  with  a  monophase 


FIG.  250. 

generator  of  the  same  cost.  Obviously  the  relative  copper 
required  may  vary  from  100,  when  the  load  is  of  lights  only, 
to  75  for  the  other  extreme  case.  With  half  lights  and  half 
motors  it  would  require  80-90  relative  copper,  according  to  the 
allowances  made  for  drop,  inductance,  etc.  In  point  of  con- 
venience it  is  very  similar  to  the  "monocyclic  "  system,  and 
like  the  latter  may  be  used  with  great  ease  in  remodeling 
monophase  systems  for  motor  work,  without  requiring  special 
generators  of  a  type  which  is  tending  to  obsolescence. 

A  natural  derivative  of  this  mixed  system  is  shown  in  Fig. 
251.  It  is  a  combination  of  Figs.  249  and  250;  a  and  b  being 
the  mains,  c  the  motor  wire  and  d  the  neutral  wire.  The  rela- 
tive copper  required  naturally  varies  with  the  proportion  of 
motors  and  lights;  36  representing  that  necessary  for  an 
approximately  eqx^al  division  under  ordinary  conditions.  Fig. 
251  may  be  compared  with  Fig.  245,  the  monocyclic  three-wire 
system.  It  is  about  the  same  in  effect  as  the  three-phase 
system  with  neutral,  having  but  two  branches  instead  of  three 
to  balance,  and  paying  for  this  privilege  with  about  20  per 
cent,  more  copper. 

There  is  thus  a  liberal  choice  of  methods  more  or  less  avail- 
able for  the  general  distribution  of  power  and  light.  Any  one 
of  them  may  prove  to  be  the  most  useful  in  particular  situa- 
tions. Now  and  then  it  may  be  worth  while  to  use  more  than! 


CENTRES  OF  DISTRIBUTION.  535 

one  of  them  in  the  same  plant,  as,  for  example,  monophase  two- 
wire  and  monophase  three-wire  or  three-phase  and  three-phase 
with  neutral. 

It  must  be  borne  distinctly  in  mind  that  one  cannot  organize 
a  large  sub-station  distribution  successfully  on  any  substantially 
two-wire  system— the  cost  of  copper  is  too  great.  If  work 
akin  to  that  of  a  large  central  station  is  to  be  done,  methods 
must  be  used  akin  to  those  which  have  proved  successful  in 


FIG.  251. 

such  work.  The  methods  of  distribution  must  be  those  which 
are  capable  of  giving  a  secondary  network  of  moderate  cost, 
easy  to  install  and  maintain.  The  use  of  alternating  current 
gives  a  great  advantage  in  the  use  of  high  tension  feeders  and 
in  efficient  methods  of  regulation,  and  there  is  at  present  no 
difficulty  in  furnishing  a  reliable  and  efficient  motor  service;  but 
to  secure  the  full  advantage  of  all  this,  one  must  cut  loose 
from  the  traditions  of  alternating  current  service.  A  trans- 
former must  be  looked  upon  not  merely  as  a  device  for  lower- 
ing the  voltage  to  a  point  available  for  direct  consumption, 
but  as  a  generator  of  extreme  simplicity  and  enormous  efficiency 
that  operates  without  attention,  can  be  started  and  stopped 
from  any  convenient  point,  and  may  be  regulated  without 
material  loss  of  energy.  That  it  receives  current  from  a 
transmission  line  instead  of  energy  of  rotation  from  a  steam 
engine  is  clear  gain  in  simplicity,  not  a  marvel  to  be  looked 
at  askance.  On  the  contrary,  the  transmission  plant  is  usually 
quite  as  manageable  and  trustworthy  as  a  steam  plant. 

Approaching  the  sub-station  from  this  standpoint,  the 
problem  of  effective  distribution  becomes  tolerably  straight- 
forward. Given  the  transmitted  energy,  it  must  be  distributed 


536  ELECTRIC   TRANSMISSION  OF  POWER. 

over  a  known  area  cheaply  and  efficiently,  with  the  smallest 
feasible  loss  of  energy  at  all  loads,  and  the  best  regulation 
attainable.  It  will  not  do  to  plead  transformer  losses  when 
the  lights  burn  dim,  or  the  depravity  of  alternating  motors 
when  they  flicker. 

First,  as  to  locating  a  sub-station.  On  general  principles  any 
station  should  be  placed  as  nearly  as  may  be  at  the  centre  of 
its  load,  and  inasmuch  as  a  transformer  station  requires  little 
space  and  makes  little  noise,  there  are  few  limitations  to  its 
position  save  the  ability  to  bring  to  it  the  transmission  lines, 
which,  being  generally  at  very  high  voltage,  w.ill  be  eyed 
cautiously  by  the  municipal  authorities.  The  main  district  to 
be  covered  is  generally  quite  definite,  and  the  next  thing  to  be 
done  is  to  reach  every  part  of  it  with  a  network  of  working 
conductors  proportioned  to  the  service.  The  nature  of  the 
wiring  will  vary,  according  to  the  system  employed;  but  the 
generally  accepted  principles  are,  save  for  inductance,  the  in- 
fluence of  which  has  already  been  considered,  the  same  that 
are  familiar  in  continuous  current  work. 

The  problem  is  to  supply  a  certain  amownt  of  energy  at  a 
given  loss  over  a  known  area,  and  the  formulae  already  stated 
give  the  key  to  the  solution.  Working  out  the  details, 
however,  is  a  somewhat  complicated  matter,  requiring  great 
judgment  and  finesse,  and  to  be  accomplished  properly  only 
by  an  experienced  engineer,  working  on  the  spot.  The 
intricacies  of  the  problem  are  too  great  to  be  treated  in  an 
elementary  treatise  like  the  present.  The  general  situation, 
however,  is  something  as  follows :  A  city,  Fig.  252.  is  to  be  sup- 
plied with  light  and  power  from  a  transmission  plant.  Let  A 
be  the  centre  of  load  at  which  the  transmission  lines  terminate. 
At  this  point  can  most  advantageously  be  located  the  reducing 
sub-station,  lowering  the  voltage  of  transmission  to  perhaps 
2,200  volts  for  feeders,  or  to  a  tenth  of  this  for  direct  supply. 
The  centre  of  load  considered  is  not  the  geographical  centre 
of  the  district  to  be  supplied,  but  the  centre  of  gravity  of  the 
load.  This  is  determined  just  as  it  the  electrical  loads  at 
various  points  were  weights  fastened  on  a  rigid  framework. 
For  example,  suppose  there  are  given  the  loads  of  Fig,  253,  five 
in  number  and  in  relative  magnitude  as  shown  by  the  figures. 
Connect  any  two  of  them,  as  i  and  2.  These  would  balance 


CENTRES  OF  DISTRIBUTION. 


537 


as  weights  at  the  point  a,  which  acts  with  respect  to  other  points 
as  if  i  and  2  were  concentrated  at  it.  Now  connect  a  and  3. 
These  weights  are  equal,  hence  the  point  of  balance  is  the 


FIG.  252. 

middle  point  of  a  3,  b,  at  which  the  weight  is  evidently  6;  b  4 
balances  at  c,  where  the  weight  is  10,  and  finally  the  whole 
system  balances  at  d,  whioh  is  the  centre  of  gravity.  The 


^«»  ' 


FIG.  253. 

points  may  be  taken  in  any  order,  but  each  line  must  be  divided 
so  that,  for  instance,  the  length  a  i,  multiplied  by  weight  i, 
shall  equal  the  length  a  2  multiplied  by  weight  2. 

The  centre  of  load  thus  found  should  be  the  centre  of  dis- 
tribution to  secure  maximum  economy  in  copper.  The  fact 
that  distribution  lines  usually  run  in  a  rectangular  street 


S38  ELECTRIC   TRANSMISSION  OF  POWER. 

system  renders  the  solution  thus  obtained  merely  approximate, 
but  it  is  nevertheless  close  enough  for  purposes  of  station 
location. 

Recurring  to  Fig.  252,  several  methods  of  arranging  the 
service  are  available.  The  simplest  is,  if  the  load  is  tolerably 
concentrated,  to  institute  a  secondary  network  about  A  so  as  to 
include  a  good  part  of  the  load  and  then  to  pick  up  the  outlying 
load  by  transformers,  placed  where  they  can  do  the  most  good, 
fed  from  high  tension  feeders.  Sometimes,  however,  there  will 
be  no  heavy  service  near  the  centre  of  load,  so  that  the  whole 
work  of  the  station  will  be  done  through  high  tension  feeders, 
each  supplying  through  its  transformers  a  more  or  less  exten- 
sive system  of  secondaries. 

As  has  already  been  pointed  out,  there  is  every  reason  for 
using  a  secondary  network,  connected  directly  to  the  reducing 
transformers,  at  the  sub-station  if  possible,  thereby  avoiding 
the  expense  of  transformers  for  a  second  reduction  in  voltage 
and  the  loss  of  efficiency  involved  in  such  a  reduction.  The 
house-to-house  transformer  distribution  should  be  shunned  as 
one  would  shun  the  plague,  if  there  is  any  expectation  of  se- 
curing an  efficient  station,  capable  of  giving  first-class  service. 

It  must  be  remembered  that  to  be  successful  a  modern  plant 
for  distributing  power  and  light  throughout  a  city  must  be 
able  to  compete  with  the  best  that  can  be  done,  not  with  the 
precarious  and  shiftless  service  of  a  dozen  years  ago. 

It  is  possible  with  a  modern  alternating  plant  to  equal  the 
best  service  given  by  a  continuous  current  central  station, 
but  the  feat  can  be  accomplished  only  by  the  study  of  cen- 
tral station  practice. 

The  sub-station  at  A,  Fig.  252,  should  be  treated,  so  far  as 
distribution  is  concerned,  as  if  the  reducing  transformers  were 
ordinary  generators.  The  transformer  units  should  be  of  the 
size  that  would  be  convenient  if  they  were  generators,  and  the 
bank  should  be  so  managed  as  to  keep  the  transformers  in  use 
as  thoroughly  loaded  as  possible.  From  the  transformer  bank 
should  run  feeders  to  the  principal  sub-centres  of  distribution 
in  the  network,  with  boosting  transformers  and  pressure 
regulators  in  such  of  the  feeders  as  require  them.  From  these 
sub-centres  pressure  wires  should  run  back  to  the  station  for 
the  guidance  of  the  operator  in  charge  of  the  regulators. 


CENTRES   OF  DISTRIBUTION1.  539 

Outside  the  effective  radius  of  distribution  of  the  principal 
secondary  network  will  come  the  independent  sub-centres 
referred  to,  with  their  high  tension  feeders  and  subsidiary  net- 
works. These  latter  should  be,  so  far  as  possible,  interlinked 
so  that,  at  times  of  light  load,  only  the  transformers  actually 
needed  shall  be  in  service.  If  secondary  pressure  wires  are 
brought  home  from  the  subsidiary  networks  all  the  regulation 
can  be  done  on  the  high  tension  feeders,  thereby  giving  equally 
good  service  all  over  the  plant.  Most  continuous  current  sta- 
tions extend  their  lines  far  beyond  the  radius  that  is  econom- 
ical for  low  tension  currents,  and  often  have  to  depend  on 
boosters  with  feeders  worked  at  a  heavy  loss  for  service  in 
the  outlying  districts.  With  an  alternating  system  this  diffi- 
culty is  avoided,  and  the  loss  in  transformers  and  regulators 
is  far  less  than  that  incurred  with  boosters  and  long  low  ten- 
sion feeders. 

As  for  the  motor  service  in  such  a  system,  it  should  be 
treated  by  common  sense,  as  it  would  be  in  a  central  station 
distributing  continuous  current. 

Alternating  motors,  polyphase  or  other,  can  be  connected 
to  the  secondary  mains  up  to  the  point  at  which  their  demands 
for  current  become  burdensome.  At  that  point  the  mains 
must  be  reinforced  or  special  feeders  run,  just  as  would  be  the 
case  with  continuous  current  motors.  The  only  difference  is 
that  produced  by  the  so-called  idle  current  in  the  alternating 
motors,  which  simply  means  that  the  point  in  question  is 
reached  a  little  sooner  than  with  continuous  current  motors. 
In  practice  this  difference  need  not  be  enough  to  be  of  serious 
moment  in  plants  having  the  ordinary  proportions  of  lights 
and  motors.  In  case  of  large  motor  plants  in  which  the  service 
is  severe,  the  use  of  special  high  tension  feeders  will  relieve 
the  trouble  that  might  be  experienced  with  the  lights,  but  this 
expedient  is  one  to  which  recourse  would  seldom  have  to  be 
taken  on  a  large  scale. 

The  greatest  difficulty  in  such  sub-station  distribution  is,  as 
has  been  already  indicated,  the  arc  lighting.  At  present  the 
alternating  arc  lamp  is  hardly  adequate  to  meet  all  conditions, 
although  it  is  coming  gradually  into  more  and  more  extended 
and  successful  use. 

In  cases  where  power  is  to  be  supplied  for  railway  purposes, 


540  ELECTRIC   TRANSMISSION   OF  POWER, 

there  are  few  difficulties  in  the  way.  Existing  railway 
generators  can  readily  be  utilized  by  driving  them  from 
synchronous  motors.  This  is  the  method  employed  in  the 
long  transmission  to  Sacramento,  Cal.,  and  elsewhere  not 
infrequently.  Where  the  utilization  of  the  old  machine  is  not 
important,  or  in  new  plants,  the  tendency  is  to  use  the  rotary 
converter,  which  has  been  already  fully  discussed.  Such 
apparatus  was  first  put  into  extensive  use  in  the  Portland 
(Ore.)  transmission  plant,  and  is  now  largely  and  very  suc- 
cessfully employed  elsewhere.  Continuous  current  for  other 
purposes  may  be  obtained  with  ease  by  the  various  methods 
described  in  Chapter  VII.  A  very  instructive  example  of 
recent  practice  in  sub  station  distribution  may  be  found  in 
Salt  Lake  City,  Utah.  This  city  is  supplied  with  electric 
power  from  three  transmission  plants,  the  general  location  of 
which  is  shown  in  Fig.  254.  These  plants  were  started  inde- 
pendently, but  later  were  consolidated  with  the  local  lighting 
interests  and  are  operated  together.  The  Big  Cottonwood 
plant,  started  in  1896,  contains  four  450  KW.  three-phase 
generators,  and  has  a  double  10,000  volt  circuit  fourteen  miles 
long  into  Salt  Lake  City.  The  Ogden  plant,  started  the  suc- 
ceeding year,  has  five  750  KW.  three-phase  generators,  at  2300 
volts,  at  which  pressure  energy  is  supplied  in  the  city  of  Ogden. 
The  rest  of  the  output  is  raised  to  16,000  volts  and  sent  into 
Salt  Lake  City  over  a  pair  of  circuits  36)^  miles  long. 

The  third  plant,  that  of  the  Utah  Power  Co.,  is  like  the 
first,  in  the  Big  Cottonwood  Canon,  but  is  two  miles  nearer 
the  city,  and  contains  two  750  KW  two-phase  generators,  with 
a  two-phase-three-phase  raising  bank  of  transformers  to  16,000 
volts,  feeding  duplicate  three-phase  circuits. 

The  Utah  plant,  with  one  generator  and  line  of  the  Big 
Cottonwood  plant,  are  put  in  parallel  on  the  high  tension  side, 
and  run  two-phase  rotaries  in  a  sub- station  near  the  centre  of 
the  city.  This  sub-station  supplies  power  to  the  electric  rail- 
way system,  and  is  entirely  separate  from  the  lighting  distri- 
bution. 

The  Ogden  lines  and  the  remaining  Big  Cottonwood  line  are 
put  in  parallel  on  the  low  tension  23oo-volt  side  at  a  centrally 
located  sub-station  devoted  to  lighting  and  power.  From  this 
sub-station  is  carried  out  a  system  of  three-phase  primary 


CENTRES   OF  DISTRIBUTION. 


541 


TTUh  Ueht  sod  Povei  Co.  Lto« 
Utah  County  Light  ud  Poiwr 
Co. 

N USD'S  Line 
Jordan  Narrow.  Uue 


FIG.  254. 


542 


ELECTRIC    TRANSMISSION  OF  POWER. 


feeders  and  mains  serving  the  entire  city.  This  network  is 
well  shown  in  Fig.  255.  The  primaries  are  connected  in  mesh, 
but  the  secondaries  have  the  star  connection  with  neutral, 
forming  a  regular  three-phase  4-wire  distribution,  with  115 


2000  Toll  Feeders          3  wire  — — 
2000  Volt  Mains     3  wire 2  wire: 


DDDDDDDaDDD 
DDDDDDDnDDD 
DDDDDDDDDDD 


.    255. 


volts  between  the  neutral  and  either  phase-Wire.  Motors  are 
connected  to  the  three-phase  wires,  giving  about  200  volts,  and 
all  motors  over  10  HP  are  put  on  transformers  of  their  own. 
As  appears  from  the  cut,  Fig.  255,  the  primary  network  is 
quite  symmetrically  arranged  with  reference  to  the  exten- 
sion of  service. 

The  secondary  service  is  developed  into  a  systematic  net- 
work  of  mains,  well  shown  in  Fig.  256.  Where  the  service  is 
dense  there  is  a  regular  4-wire'network.  Each  block  is  served 


CENTRES  OF  DISTRIBUTION. 


543 


by  two  groups  of  three  transformers  at  the  opposite  corners, 
from  which  secondary  mains  are  carried  around  the  block  and 
tied  by  fuse  boxes  to  the  secondary  mains  of  adjacent  blocks. 


Secondary  Mains  -    4  wire 
Secondary  Mains  -    2  wire 

Transformers  represented  by  dott 

Transformer  not  shown  on  4  wire  mains. 


in^jigi  OpTTtt 

j^&nnnE 
^iiSrijTTi  ii 

i^soaui?. 

iDDpannr 


*r 


) dnDDDDDDDDOn 

/BanDnananoDD 
/ DP n a D n D D D P D P P 
[  pnaapapnanDaD,..  ' 1 

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FIG.  256. 

Where  service  is  less  dense,  as  in  residence  districts,  the  first 
step  is  to  put  in  a  2-wire  secondary  main  from  the  trans- 
formers, consisting  of  a  neutral  and  one  phase-wire,  street 
being  balanced  against  street  in  this  light  service.  Then 
when  business  demands,  the  other  phase  wires  are  carried  into 
the  street,  lights  balanced  upon  them,  and  the  completed 
4-wire  system  is  then  tied  to  the  network  already  completed. 

Commercial  arc  lighting  is  by  constant  potential  alternating 
arcs,  of  which  some  500  are  in  use  in  Ogden  and  Salt  Lake 


544  ELECTRIC   TRANSMISSION  OF  POWER. 

City.  Street  lighting  is  at  present  supplied  from  continuous 
current  series  arc  machines  driven  by  synchronous  motors. 
These  motors  are  located  in  an  old  electric  light  station  near 
the  sub -station,  and  can  be  driven  as  generators  in  case  of 
need,  while  the  sub-station  itself  has  a  small  reserve  steam 
plant  and  generator  equipment.  The  synchronous  motors  are 
useful  in  regulating  the  voltage  at  Salt  Lake  City,  being 
capable  of  accomplishing  a  variation  of  10  per  cent,  when 
the  lines  are  heavily  loaded. 

This  scheme  of  sub-station  distribution  is  admirably  con- 
ceived, and  works  out  very  simply  and  neatly.  The  trans- 
mission system  itself  is  decidedly  complex,  owing  to  the 
various  and  diverse  power  houses,  but  it  works  well  and  has 
done  excellent  service.  It  is  interesting  to  note  that  no 
trouble  is  experienced  in  running  these  distant  and  diverse 
plants  in  parallel.  At  light  load  there  is  some  interchange  of 
current,  but  at  heavy  loads  everything  settles  down  to  business. 

The  three  stations  are  connected  by  telephone,  and  by  a 
little  intercommunication  the  generators  can  be  put  in  parallel 
in  the  ordinary  manner  either  at  a  station  or  at  the  sub-station 
in  Salt  Lake  City.  The  record  of  the  system  for  continuity 
of  service  has  been  good,  and  it  is  worth  noting  that  most 
instances  of  trouble  on  the  lines  have  been  due  to  malicious 
interference,  such  as  shooting  off  insulators  and  throwing 
things  across  the  lines.  Altogether  the  system  is  a  notable 
instance  of  the  flexibility  and  convenience  of  modern  power 
transmission  methods,  as  well  as  a  good  example  of  a  system- 
atic and  logical  development  of  the  distribution  system.  As 
the  service  grows  various  refinements  will  doubtless  suggest 
themselves,  but  the  system  is  correctly  started  and  there  will 
be  little  work  to  undo.  It  is  in  striking  contrast  with  some 
transmission  systems  which  could  be  named,  in  which  the  oper- 
ators, less  skilled  in  dealing  with  modern  methods,  have 
blundered  around  trying  to  give  good  service  in  an  unsystem- 
atic and  helter-skelter  fashion,  getting  deeper  into  trouble  at 
every  jump,  and  then  blaming  the  state  of  the  art  for  the 
results  of  their  own  lack  of  discretion. 

The  most  delicate  and  important  work  in  connection  with 
heavy  sub-station  service  is  that  involved  in  the  proper  regu- 
lation of  the  voltage.  The  sub-station  receives  its  supply  of 


CENTRES  OF  DISTRIBUTION. 


545 


energy  often  from  a  long  transmission  line  in  which  there  is 
considerable  drop,  to  say  nothing  of  that  encountered  in  the 
generators  and  two  banks  of  transformers. 

It  must  distribute  this  energy  throughout  a  complicated 
network,  so  that  the  variations  in  pressure  at  the  lamps  shall 
not  exceed  two  or  three  volts  at  the  outside.  This  is  never  an 
easy  task — it  tries  the  ingenuity  even  of  the  best  central 
station  engineers. 

In  connection  wifeh  a  transmission  plant  probably  the  best 


FIG.  257. 

plan  is  to  divide  the  regulation  into  two  stages:  first  that  con- 
cerned with  the  transmission  proper,  and  second  that  concerned 
with  the  distribution.  By  compounding  the  generators,  or  by 
hand  or  automatic  regulation  of  generators  having  good  inher- 
ent regulation,  it  is  certainly  possible  to  hold  the  voltage  closely 
constant  up  to  the  primary  terminals  of  the  reducing  trans- 
formers. In  large  alternating  generators  compounding  is 
seldom  or  never  attempted,  and  in  many  cases  the  sole 
reliance  is  hand  regulation,  which  is  by  no  means  to  be  despised 
in  the  absence  of  other  means. 

Within    the    last   few   years    several    automatic    regulators 
capable  of  giving  excellent  service  have  been  brought  out,  and 


546 


ELECTRIC   TRANSMISSION   OF  POWER. 


they  are  coming  into  somewhat  extensive  use.  Fig.  257 
shows  in  diagram  the  principles  of  operation  of  the  latest 
form  of  the  Chapman  regulator,  which  is  an  excellent  type  of 
such  apparatus. 

The  fundamental  idea  is  exceedingly  simple — a  rheostat  in 
the  fields  of  the  generator,  with  its  contact  arm  actuated  by  a 
pair  of  powerful  solenoids,  of  which  one  or  the  other  is  thrown 
into  action  by  a  voltmeter  relay.  In  the  actual  working  out 


FIG    258. 

of  the  device  there  is  room  for  great  designing  skill.  In  Fig. 
257  the  rheostat  segments  are  arranged  in  the  arc  of  a  circle, 
with  the  contact  arm  capable  of  rotation  in  a  vertical  plane 
above  them.  This  contact  arm  makes  a  smooth  spring  con- 
tact with  the  rheostat  segments,  and  is  actuated  directly  by 
the  solenoids  connected  with  the  exciter  circuit  through  the 
relay.  The  plunger  in  one  solenoid  is  extended  into  the 
piston  of  an  adjustable  dashpot,  so  that  the  contact  arm  can 
be  made  to  move  quickly  or  slowly,  as  is  desired. 
The  relay  is  supplied  with  current  from  a  potential  trans- 


CENTRES  OF  DISTRIBUTION. 


547 


former  across  the  main  circuit,  and  in  the  form  shown  it  also  has 
a  subdivided  compound  winding  fed  from  a  current  transformer 
having  its  primary  in  the  main  leads.  Such  current  trans- 
formers are  commonly  used  with  ordinary  ammeters  employed 
on  circuits  of  2500  volts  and  upward.  The  "compound 
switch"  enables  the  relay  to  be  adjusted  so  as  to  shift  the 


FIG.  259. 

rheostat  in  such  wise  as  to  hold  the  voltage  constant  at  the 
end  of  the  transmission  line  instead  of  at  the  terminals  of  the 
machine. 

The  solenoids  are  provided  with  short-circuited  windings  in 
addition  to  the  working  windings,  to  prevent  sparking  at  the 
relay  contacts,  which  they  do  most  effectively.  The  relay 
will  operate  on  less  than  a  volt  variation  on  a  no-volt  circuit, 
and  the  regulator  working  as  it  does  directly  on  the  generator 
fields  can  and  does  hold  the  voltage  very  close  to  normal 
under  any  ordinary  variations  of  load*  Figs.  258  and  259  give 
an  excellent  idea  of  the  results  actually  attained.  The  former 
shows  the  variations  in  voltage  recorded  prior  to  the  installa- 
tion of  the  regulator.  They  are  of  course  outrageously  bad, 


548 


ELECTRIC   TRANSMISSION-  OF  POWER. 


and  could  have  been  greatly  bettered  by  a  little  care  in  hand 
regulation.  But  the  diagram,  Fig.  258,  taken  after  the  Chap- 
man regulator  was  install  jd,  shows  the  voltage  held  admirably 
steady  throughout  the  run. 

Several  other  automatic  regulators  are  on  the  market  and  are 
capable  of  doing  excellent  service.     An  earlier  but  still  very 


FIG.  260. 

effective  form  of  the  Chapman  regulator  is  made  by  the  Belknap 
Motor  Co.,  and  for  use  with  continuous  current  generators  or 
with  the  exciters  of  alternators  a  still  simpler  device,  the  Tir- 
rell  regulator,  has  made  an  excellent  record.  None  of  these  reg- 
ulators are  arranged  automatically  to  take  care  of  the  line  drop 
when  the  power  factor  varies  considerably,  but  they  are  amply 
sufficient  to  provide  for  the  general  regulation  up  to  the  sub- 
station, at  which  point  it  may  be  taken  up  as  a  separate  prob- 
lem. This  residual  regulation  ordinarily  consists  of  the  drop  in 
the  reducing  transformers,  which  should  be  not  over  2  per  cent. ; 
the  drop  in  the  feeders  and  secondary  mains;  in  high  tension 
feeders  and  transformers  when  employed;  and  finally  in  the 
house  wiring.  These  losses  will  aggregate  generally  less  than 


CENTERS  OF  DISTRIBUTION.  549 

10  per  cent.,  and  are  best  cared  for  in  the  sub-station.  As  the 
variations  in  load,  and  hence  in  loss,  are  generally  rather  slow, 
this  regulation  should  be  accomplished  without  difficulty.  In 
some  cases  it  may  be  advantageously  reduced  in  amount  by 
carrying  the  primary  regulation  through  to  the  secondary 
terminals  of  the  reducing  transformers. 

However  this  may  be,  the  regulation  of  the  voltage  on  the 
secondary  lines  must  be  carried  out  with  the  utmost  care. 
The  apparatus  employed  for  this  purpose  is  both  very  simple 
and  exceedingly  efficient.  It  is  in  every  case  a  transformer 
arranged  to  give  a  variable  ratio  of  transformation  and  adding 
its  E.  M.  F.  to  that  of  the  working  circuit. 

The  best  known  form  of  this  device  is  probably  the  Stillwell 
regulator,  which  has  for  some  years  past  been  very  successfully 


\ 


* 

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^     |         TO 

p  s  |B  LO/\O 

FIG.  261. 

used  by  the  Westinghouse  company.  It  is,  in  effect,  a  trans- 
former, from  the  secondary  coil  of  which  leads  are  brought  out 
to  terminals  so  arranged  as  to  enable  one  to  vary  the  number 
of  secondary  turns,  and  so  to  vary  the  E.  M.  F.  added  to  the 
working  circuit.  Fig.  260  shows  a  diagram  of  the  connections 
by  which  this  result  is  effected.  The  diagram  is  self-explana- 
tory, except  that  it  should  be  noted  that  the  "preventive 
coil  "  is  intended  to  avert  the  necessity  of  breaking  circuit  or 
short  circuiting  a  secondary  coil  in  passing  from  one  contact 
to  the  next,  and  that  the  reversing  switch  enables  the  regulator 
to  diminish  the  voltage  on  the  working  circuit,  which  may  now 
and  then  be  convenient.  In  the  ordinary  practice  of  the 
Westinghouse  company  this  regulator  is  installed  in  the  gener- 
ating station  and  used  to  vary  the  voltage  on  the  primary  line. 
In  sub-station  work  it  can  be  applied  either  to  the  primary  or 
secondary  side  of  the  reducing  transformers;  practically  the 
latter  is  the  working  connection.  These  regulators  are  made 


55°  ELECTRIC    TRANSMISSION  OF  POWER. 

to  have  a  range  of  action  of  10,  15  and  20  per  cent,  of  the 
working  voltage.  They  are  generally  employed  with  a  very 
ingenious  device  known  as  the  "compensator,"  the  function 
of  which  is  to  indicate  the  pressure  at  the  end  of  the  line  or 
feeder  without  the  use  of  pressure  wires.  The  principle  of  this 
is  shown  in  Fig.  261.  The  voltmeter  Fis  in  circuit  with  the 
opposed  E.  M.  Fs.  of  two  secondaries  C  and  Z>,  of  which  the 
primaries  A  and  D  are  respectively  in  series  and  in  shunt  with 
the  load.  The  voltage  of  D  is  proportional  to  the  main 
primary  E.  M.  F.,  that  of  C  to  the  primary  current  strength, 
so  that  the  difference  between  C  and  Z>,  which  shows  on  the 
voltmeter,  can  be  made  proportional  to  the  voltage  as  reduced 
by  the  drop  due  to  the  current  in  the  line.  The  compensator 
is  in  addition  provided  with  a  series  of  contacts  by  which  the 
E.  M.  F.  of  C  is  adjustable  for  any  given  percentage  of  loss  in 
the  line. 

The  practice  of  the  General  Electric  Company  is  somewhat 
different.  The  generator  is  generally  over  compounded  for  a 
fixed  loss  in  the  line  at  full  load,  or  hand  regulation  is  effected 
by  the  field  rheostat.  For  sub-station  purposes  a  variable 
transformer  is  employed  to  vary  the  working  voltage.  The 
principle  of  this  voltage  regulator  is  the  variation  of  the  induc- 
tive relation  of  primary  and  secondary  instead  of  varying  the 
number  of  secondary  turns.  The  apparatus  itself  is  made  in 
several  forms,  one  of  which,  used  in  the  Portland  (Ore.)  three- 
phase  plant,  is  shown  in  Fig.  262.  It  is  essentially  a  trans- 
former with  a  movable  secondary,  and  serves  either  to  raise  or 
lower  the  working  voltage,  as  occasion  requires.  The  gra- 
dation of  voltage  is  not  by  definite  steps,  but  by  continuous 
variation.  The  apparatus  is  made  for  substantially  the  same 
range  of  action  as  the  Stillwell  regulator  just  described,  and 
accomplishes  the  same  result.  The  General  Electric  Co.  also 
makes  a  voltage  regulator  with  a  variable  number  of  secondary 
windings. 

It  should  be  stated  that  neither  over-compounding  nor  any 
similar  device  can  deal  successfully  with  a  load  of  variable 
inductance,  such  as  is  often  found  in  motor  service.  They 
can  be  made  to  work  well  on  either  non-inductive  or  inductive 
load,  but  are  not  well  adapted  for  a  load  of  which  the  power 
factor  varies  much.  For  this  condition  nothing  has  yet 


CENTRES   OF  DISTRIBUTION.  55* 

been  devised  so  good  as  pressure  wires  combined  with  intelli- 
gent hand  regulation. 

Various  attempts  have  been  made  to  employ  pressure  wires 
in  conjunction  with  automatic  regulators,  but  none  have  yet 
met  with  very  encouraging  success.  Automatic  control  of 
alternating  current  sub-station  regulators  is  by  no  means  so 
simple  a  matter  as  pressure  regulation  applied  to  the  genera- 
tor. Apparatus  of  the  type  of  the  Stillwell  regulator  has  to  dea) 
with  fairly  large  currents,  and  the  contact  arm,  to  prevent 
undue  load  on  the  "preventive  coil,"  should  be  quickly  moved* 
from  segment  to  segment.  This  involves  various  mechanical 
difficulties  and  the  expenditure  of  some  power.  Apparatus 
like  Fig.  262  works  none  too  easily,  on  account  of  the  mag: 
netic  forces  involved.  In  fact,  it  is  safe  to  say  that  the 
problem  of  working  sub-station  regulators  automatically 
involves  the  use  of  powerful  relay  mechanism  akin  to  that 
used  for  water-wheel  governors,  although  on  a  very  much 
smaller  scale. 

No  such  apparatus  is  just  at  present  available,  although  if 
successful  it  would  be  in  considerable  demand.  Still  less 
progress  has  been  made  toward  the  development  of  an 
automatic  balancing  device  for  polyphase  circuits.  Given  a 
good  automatic  sub-station  regulator,  and  its  application  to 
preserving  accurate  balance  in  a  two-phase  or  three-phase 
distributing  system  is  an  obvious  extension  of  its  general  use. 
Balance  is  not  difficult  to  secure  with  a  little  tact  in  arranging 
the  load,  but  sometimes  when  there  is  a  particularly  heavy 
lap  load  there  will  be  sensible  unbalancing  while  this  load  is 
coming  on  and  going  off.  This  is  taken  care  of  sometimes 
by  having  certain  loads  that  can  be  switched  at  will  upon  any 
leg  of  the  circuit,  and  sometimes  pressure  regulators  are 
installed  for  manual  operation.  A  good  automatic  balancer 
and  pressure  regulator  would  often  be  of  very  considerable 
service,  but  it  is  not  yet  forthcoming.  It  must  not  be 
supposed  that  the  lack  of  it  is  a  very  grave  deficiency,  since 
practically  all  ordinary  central  station  regulation  is  manual 
save  in  so  far  as  it  can  be  accomplished  by  over-compounding 
the  generators. 

The  devices  just  described  are  amply  competent  to  furnish 
very  exact  regulation  for  sub-station  purposes.  Its  complete- 


55* 


ELECTRIC   TRANSMISSION  OF  POWER. 


ness  depends  in  the  last  resort  on  the  skill  with  which  the  dis- 
tributing system  is  designed.  If  this  is  carefully  done,  the 
sub-station  regulation  should  hold  the  voltage  within  very 
narrow  limits  clear  up  to  the  lamps. 

As  regards  the  best  system  of  transmission  to  employ  in 
connection  with  heavy  sub-station  work,  there  is  naturally  a 
wide  diversity  of  opinion.  In  the  author's  judgment  there  is 
at  present  no  distributing  system  for  large  sub-station  work 
so  generally  advantageous  as  the  three-phase  distribution  with 


FIG.  262. 


neutral  wire  shown  in  Fig.  249.  It  is  remarkably  free  from 
trouble  as  regards  balancing,  and  extraordinarily  economical 
of  copper.  With  further  advance  in  the  development  of  single- 
phase  alternating  motors,  the  single-phase  three-wire  system 
shown  in  Fig.  242  will  do  admirable  work  when  the  motor 
service  is  rather  light.  The  diphase  system  has  been  installed 
in  some  central  stations  and  the  "monocyclic"  in  others,  so 
data  will  eventually  be  available  regarding  each  of  these  sys- 
tems, but  there  is  little  reason  to  expect  as  good  general  results 
as  could  be  obtained  by  the  systems  mentioned  above.  Di- 
phase, monccyelic,  and  the  Dresden  three-phase  systems 


CENTRES  OF  DISTRIBUTION.  553 

are,  however,  very  much  easier  to  adapt  to  the  circuits  of 
present  stations  than  is  the  three-phase  system  with  neutral 
wire. 

When  a  large  part  of  the  output  of  a  transmission  plant  is 
required  for  railway  work  and  other  motor  service  of  extreme 
severity,  and  a  lighting  system  is  also  to  be  operated,  it  is  a 
wise  precaution  to  work  the  two  services  normally  over  sep- 
arate lines  and  from  separate  generators,  as  is  done  in  the 
Salt  Lake  City  system  just  described.  Otherwise  the 
variations  of  load  may  be  so  great  and  so  rapid  that  no  care 
in  regulation  could  prevent  serious  fluctuations  in  voltage. 
A  small  railway  load  and  all  ordinary  motor  service  can  be 
worked  from  the  same  circuits  as  lamps  without  much  difficulty. 
These  limitations  are  not  peculiar  to  transmission  plants — no 
Edison  station,  for  instance,  would  dare  to  attempt  working  a 
low  voltage  conduit  railway  from  its  lighting  mains.  In  these, 
as  in  many  similar  matters,  a  little  common  sense  will  prevent 
serious  mistakes  and  show  the  necessity  of  working  every 
system  so  as  to  obtain  the  best  possible  results  and  not  to 
discover  what  it  will  endure  without  giving  intolerably  bad 
service.  Of  late  storage  battery  auxiliaries  have  often  been 
suggested,  and  sometimes  have  been  employed,  in  connec- 
tion with  power  transmission  plants.  Some  reference  has 
already  been  made  to  storage  in  Chapter  II,  but  the  matters 
here  to  be  considered  are  of  a  different  character.  In  trans- 
mission work  a  battery  may  be  used  for  two  entirely  distinct 
purposes.  In  the  first  place  it  may  be  used,  as  it  sometimes  is 
in  steam  driven  stations,  for  the  purpose  of  storing  energy  at 
times  of  light  load  to  be  used  in  making  up  deficiency  of  power 
at  times  of  heavy  load. 

In  steam  driven  stations  the  installation  of  a  battery  effects 
a  considerable  economy  by  enabling  the  engines  to  be  run 
at  all  times  at  the  points  of  maximum  economy,  and  an 
additional  saving,  in  first  cost,  by  reducing  the  capacity 
of  the  steam  plant  and  generators  required.  The  condi- 
tions of  economy  depend  mainly  upon  local  circumstances, 
but  a  material  saving  can  be  made  in  many  instances  by  using 
the  battery. 

In  hydraulic  practice  the  case  is  different.  In  the  average 
water  power  plant  the  main  hydraulic  works  should  generally 


554  ELECTRIC    TRANSMISSION  OF  POWER. 

be  installed  for  the  full  available  capacity,  save  in  the  few 
instances  when  a  partial  fall  can  be  economically  utilized.  As 
a  rule  the  dam  will  be  substantially  the  same  for  a  partial 
development  as  for  a  complete  one,  and  the  latter  can  be 
carried  out  more  cheaply  at  the  start  than  when  added  as 
patchwork  later.  Consequently  there  is  seldom  or  never  any 
saving  in  installing  a  costly  battery  subject  to  heavy  depreci- 
ation in  order  to  avert  the  first  cost  of  a  larger  plant.  Further, 
the  loss  of  energy  in  the  battery  is  much  greater  than  the  loss 
ordinarily  incurred  in  the  line  at  full  load,  so  that  the  total 
saleable  power  for  a  given  first  cost  would  in  nearly  every  case 
be  reduced  by  installing  a  battery.  The  one  case  in  which  a 
battery  can  advantageously  be  used  in  connection  with  power 
transmission  for  the  purpose  indicated,  is  that  in  which  the 
total  hydraulic  power  available  is  actually  insufficient  to  carry 
the  required  maximum  load.  Storage  may  then  be  very 
advantageous,  since  it  enables  the  unutilized  power  at 
light  load  to  be  applied  to  the  peak.  Especially  will  it  be 
advisable  when  the  peak  is  high  and  the  load  factor  rather 
poor,  under  which  conditions  a  battery  may  raise  the  possible 
maximum  output  by  30  to  50  per  cent.,  sometimes  even  a 
little  more. 

The  second  use  of  a  battery  is  as  a  reserve  to  tide  over  a 
brief  breakdown.  The  question  of  reserve  against  accident  in 
transmission  work  is  always  a  troublesome  one.  In  the 
author's  opinion  the  need  of  such  reserve  located  in  the  sub- 
station is  greatly  overestimated.  Experience  clearly  indicates 
that  of  the  interruptions  of  service  occurring  on  the  system  of 
a  transmission  plant  with  sub-station  distribution,  only  a  very 
small  minority  occur  on  the  transmission  line  proper.  The 
distribution  lines  throughout  an  average  city  are  peculiarly 
exposed  to  interruption  from  limbs  of  trees,  which  in  residence 
streets  can  never  be  adequately  trimmed;  from  the  fall  of 
foreign  wires;  from  necessary  cutting  off  in  case  of  fire,  and 
from  other  causes.  A  high  voltage  transmission  is  neither  more 
nor  less  likely  to  encounter  trouble  on  its  distributing  system 
than  an  ordinary  central  station.  So  far  as  these  causes 
of  trouble  go,  the  transmission  plant's  sub-station  is  exactly 
on  a  par  with  any  other  central  station  in  requiring  special 
precautions.  Now  while  central  stations  always  should  have 


CENTRES   OF  DISTRIBUTION.  555 

more  or  less  reserve  apparatus  to  use  in  case  of  breakdown, 
it  is  not  required  on  account  of  possible  trouble  on  the  line 
except  as  such  trouble  may  injure  apparatus.  A  short 
circuit  on  the  feeding  system  will  not  be  removed  by  the 
presence  of  a  spare  engine  and  dynamo  in  the  station.  Hence 
the  need  of  reserve  in  the  sub  station  of  a  power  transmission 
system  bears  relation  simply  to  the  accidents  which  may  affect 
continuity  of  service  as  regards  the  main  transmission  line, 
and  particularly  accidents  producing  more  than  momentary 
interruptions.  Such  accidents  are  very  rare  on  properly 
designed  and  erected  lines,  and  save  on  very  long  lines  of 
which  the  cost  is  a  considerable  part  of  the  total  cost  of  the 
system,  it  is  generally  true  that  a  fraction  of  the  cost  of  a 
complete  reserve  plant  at  the  sub-station  would  provide  a 
duplicate  line  so  guarded  that  reserve  apparatus  would  be 
practically  needless.  With  well-built  duplicate  pole  lines  and 
proper  switching  arrangements,  serious  trouble  on  the  lines, 
save  under  conditions  which  would  also  paralyze  the  service 
on  the  distributing  system,  and  thus  cripple  the  plant  in  any 
event,  becomes  almost  impossible. 

Sometimes,  however,  a  partial  auxiliary  plant  is  extremely 
useful,  but  it  is  rather  for  its  convenience  in  case  of  repairs 
to  apparatus  at  the  generating  station  or  sub-station  than  as  a 
safeguard  to  the  main  line.  In  working  a  large  sub-station  a 
storage  battery  may  be  of  considerable  use  in  this  way,  partic- 
ularly if  the  system  is  being  pushed  near  to  its  capacity.  It 
is  decidedly  not  good  policy,  however,  to  use  a  battery  unless 
the  station  is  upon  a  scale  large  enough  to  warrant  the 
employment  of  an  especial  man  skilled  in  handling  batteries 
and  unburdened  with  other  duties.  Charged  and  discharged 
through  motor-generators  or  rotaries,  a  storage  battery  can 
be  put  into  service  on  a  moment's  notice,  and  is  far  less  trouble- 
some to  keep  up  than  any  other  auxiliary  for  temporary  use. 

In  some  localities  a  generator  coupled  to  a  gas  or  oil  engine 
makes  an  admirable  auxiliary.  Such  engines  can  now  be 
obtained  of  large  output  and  very  high  economy,  and  form  a 
reserve  almost  as  convenient  as  a  battery.  Steam  reserves 
are  not  large  in  first  cost,  unless  high  economy  in  operation  is 
attempted,  but  cannot  be  put  quickly  into  action  unless  the 
fires  are  kept  banked,  which  is  a  very  considerable  expense. 


556  ELECTRIC    TRANSMISSION  OF  POWER. 

However,  by  keeping  a  banked  fire  under  threatening  climatic 
conditions  the  reserve  can  be  ready  when  it  is  likely  to  be 
needed,  and  if  apparatus  needs  repair  there  is  generally  notice 
enough  given  to  get  steam  up.  Power  of  quick  firing  is  of 
great  importance  in  boilers  for  an  auxiliary  plant,  and  with 
tactful  treatment  a  steam  reserve  is  probably  the  most 
satisfactory  for  plants  of  moderate  size. 


CHAPTER  XV. 

THE   COMMERCIAL    PROBLEM. 

POWER  transmission  is  of  little  avail  if  it  does  not  pay, 
and  the  chances  of  commercial  success  form  the  first  subject 
of  investigation  in  the  development  of  any  power  transmission 
enterprise.  Reduced  to  its  lowest  terms,  the  question  pre- 
sents itself  thus:  Can  I  profitably  furnish  power  at  a  price 
which  will  enable  me  to  undersell  the  current  cost  of  power 
production?  Evidently  this  question  cannot  be  answered 
a  priori,  but  must  be  thoroughly  investigated  in  each  par- 
ticular case. 

The  first  thing  to  be  determined  is  the  existence  of  a 
sufficient  market,  the  second  thing  is  the  price  current  in  this 
market.  It  is  not  difficult  to  find  out  the  gross  amount  of 
power  used  in  a  given  region,  but  it  is  exceedingly  hard  to 
discover  the  real  cost  of  production.  Even  if  all  men  were 
strictly  veracious  it  is  a  fact  that  very  few  users  of  power 
have  any  clear  idea  of  what  they  pay  for  it.  Coal  bills  and 
wages  are  tangible  and  men  realize  them,  but  interest,  depre- 
ciation, repairs,  miscellaneous  supplies,  water,  taxes,  insurance 
and  incidentals,  are  seldom  rigorously  charged  up  to  the 
power  account,  and  these  are  large  items  when  power  is  used 
irregularly. 

Further>  the  cost  per  HP  is  often  computed  from  the  nominal 
HP  of  the  engine,  without  exact  knowledge  of  the  real  average 
yearly  load.  Hence  people  often  think  that  they  are  produc- 
ing power  at  $15  or  $20  per  HP  per  year  when  the  real  cost  is 
$30  to  $50. 

The  most  exhaustive  researches  as  yet  made  on  this  subject 
are  those  of  Dr.  C.  E.  Emery.  The  accompanying  table 
gives  a  summary  of  his  results,  based  on  500  net  HP  delivered 
for  10  hours  per  day,  308  days  in  the  year.  The  power  is 
supposed  to  be  derived  from  a  single  engine  worked  continu- 
ously at  its  normal  capacity.  These  figures  represent  results 

557 


558 


ELECTRIC   TRANSMISSION-  OF  POWER. 


much  better  than  are  generally  reached  in  practice,  since  most 
engines  are  not  worked  continuously  at  full  load.      In  a  large 


KIND  OF  ENGINE. 

COAL 

$2  PER  T. 

COAL 

$3  PER  T. 

COAL 

$4  PER  T. 

COAL 

$5  PER  T. 

Simple  high  speed  

$20  8  1 

Sl6  17 

$42  54 

$48.00 

Simple  low  speed 

28  46 

•J  \     2O 

OQ    QA 

AC  67 

Simple  low  speed,  condensing.. 
Compound     condensing,      low 
speed 

22.82 
21  Q7 

26.77 

2C  ca 

30.73 

2Q  OQ 

34.69 

•32    61? 

Triple    expansion   condensing, 
low  speed  

22  ^5 

25  ^2 

28  28 

qi  2"> 

majority  of  cases  the  real  cost  exceeds  that  given  in  the  table 
by  something  like  50  per  cent.,  even  for  engines  of  similar  size. 
For  the  rank  and  file  of  small  engines  used  for  miscellaneous 
manufacturing  purposes,  cheaply  built  and  generally  under 
loaded,  the  tabular  figures  should  be  just  about  doubled.  In 
regions  where  coal  is  unusually  dear  the  cost  in  units  of  50  HP 
and  upward  may  range  from  $100  to  $150  per  HP  year  for  a 
10  hour  day. 

In  units  under  50  HP  one  is  very  unlikely  to  find  the  HP 
year,  reckoned  on  the  above  basis  of  10  hours  per  day,  costing 
less  than  $50,  even  with  coal  as  low  as  $2  per  long  ton.  These 
are  the  facts  in  the  case;  the  fancies  will  be  duly  appreciated 
if  one  canvasses  for  electric  power.  Not  more  than  one  man 
in  six  knows  and  will  admit  that  his  power  is  costing  him  as 
much  as  the  table  would  indicate.  The  process  of  reasoning 
(so  called)  is  often  about  as  follows:  "I  paid  for  my  engine 
and  boiler  house  when  I  built  the  factory,  and  I  do  not  propose 
to  charge  my  engine  rent.  It  has  been  running  ten  years  and 
is  just  as  good  now  as  it  ever  was;  has  not  depreciated  for  my 
purpose  a  cent.  If  any  repairs  are  needed,  the  engineer  and 
one  of  my  men  have  made  them  and  they  haven't  cost  me  any- 
thing but  my  material.  My  fireman  I  have  to  have  anyhow, 
for  I  heat  by  steam,  and  my  taxes  and  insurance  I  have  to  pay 
anyhow:  that  is  a  200  HP  engine;  my  coal  cost  me  $2,450 
last  year  and  oil  and  stuff  $70.  I  pay  my  engineer  $60  a  month; 
that's  $16.20  per  horse-power  per  year;  if  you  can  furnish 
electric  power  for  $15  per  year  perhaps  we  can  trade."  This 


THE   COMMERCIAL   PROBLEM.  559 

theme,  with  variations,  is  familiar  to  anyone  who  has  had 
practical  experience  in  power  transmission  work,  and  although 
the  more  intelligent  and  able  class  of  manufacturers  are  quite 
too  keen  not  to  see  the  facts  when  properly  presented,  a  cer- 
tain amount  of  this  ignorant  short-sightedness  is  always  met 
in  investigating  the  power  market. 

With  a  working  year  as  above  of  3,080  hours  the  cost  of 
steam  power  is  actually  very  seldom  as  low  as  i  cent  per  HP 
hour,  and  in  units  below  100  HP  is  not  very  often  below 
2  cents.  In  units  of  less  than  20  HP  it  is  quite  certain  to 
be  5  cents  or  more.  These  figures  are  based  on  continuous 
working.  If  the  use  of  power  is  intermittent,  the  cost  per  HP 
hour  is  greatly  increased,  by  an  uncertain  but  always  large 
amount,  depending  on  the  nature  of  the  service.  For  highly 
intermittent  service  gas  engines  are  undoubtedly  cheaper  than 
steam,  and  in  ordinary  units  the  cost  of  operating  these  is 
seldom  less  than  10  cents  per  HP  hour  of  use.  Used  continu- 
ously at  full  load  or  thereabouts  the  gas  or  petroleum  engine 
is  the  most  formidable  competitor  of  electric  motors,  since 
the  actual  cost  of  fuel  is  low — from  2  to  5  cents  per  HP  hour — 
and  the  attendance  required  is  trifling.  Such  engines,  how- 
ever, are  high  in  first  cost  and  are  very  inefficient  at  low 
loads,  besides  being  subject  to  relatively  large  depreciation. 

These  peculiarities  are  well  shown  in  a  recent  test  of  a  6  HP 
gas  engine  in  which  the  following  facts  appeared:  The  cost 
of  operation,  including  maintenance,  was  at  full  load  41  cents 
per  hour,  and  at  no  load  20  cents  per  hour;  the  cost  of  gas 
being  $1.70  per  M  feet. 

We  may  easily  find  from  this  the  cost  of  power  under  given 
circumstances  of  use;  $10  per  HP  per  year  may  fairly  be 
charged  up  to  interest  and  depreciation.  Suppose  now,  power 
is  used  for  10  hours  per  day  308  days  in  the  year,  the  engine 
being  fully  loaded  all  the  time.  The  cost  can  be  made  up  as 
follows  for  6  HP: 

3,080  hours  @  41  cents,      .         .         .       =    $1,262.80 

Interest  and  depreciation,  .         .       =  60.00 

Total  cost,  .         .         .         .      —    $1,322.80 

Cost  per  HP  hour  =  7.15  cents,  of  which  the  interest  and 
depreciation  amounts  to  but  0.31  cents  per  HP  hour. 


560 


ELECTRIC   TRANSMISSION  OF  POWER. 


Second,  suppose  the  engine  is  in  full  use  3  hours  per  day, 
and  running  idle  the  rest  of  the  time,  or  is  in  equivalent  partial 
use  for  10  hours.  We  then  have 


924  hours  @  41  cents, 
2,156      "      "    20 
Interest  and  depreciation, 


.  =  $378.81 
.  =  431-20 
.  =  60.00 


$870.01 

This  is  12.08  cents  per  HP  hour  actually  used,  and  is  a  fair 
type  of  present  practice  as  gas  engines  are  generally  used. 
It  will  hold  for  the  average  engine  used  for  small  power  pur- 
poses. In  regular  running  such  engines  consume  from  25  to 
35  cubic  feet  of  average  illuminating  gas  per  brake  HP  and, 
when  running  light,  take  about  half  as  much  gas  as  at  full  load. 
In  careful  experimental  running  these  results  can  be  bettered 
10  to  20  per  cent.,  but  in  regular  work  and  with  only  ordinary 
care,  the  gas  consumption  given  is  correct. 

Petroleum  engines  give  rather  less  fuel  expense,  but  lose  in 
extra  care  and  repairs  nearly  or  quite  all  the  gain  in  fuel. 

These  figures  must  not  be  understood  as  applying  to  large  gas 
engines  of  100  HP  and  upward,  worked  on  cheap  "  producer" 
or  fuel  gas.  It  is  reasonably  certain  that  such  engines  give  re- 
sults better  than  any  save  the  most  economical  steam  engines, 
if  worked  at  or  near  full  load.  In  the  small  sizes  above  consid- 
ered the  gas  engine  is  a  considerably  cheaper  source  of  power 
than  steam  engines,  probably  by  not  less  than  30  per  cent. 

In  a  general  way  we  may  summarize  these  facts  regarding 
cost  of  power  as  follows,  coal  being  taken  at  $3  per  ton: 


KIND  OF  ENGINE. 

COST  PER  HPH,  io- 
HOUR  DAY, 
FULLY  LOADED. 

COST  PER  HPH,  IN- 
TERMITTENT USE, 
PARTIAL  LOAD. 

Large  compound  cond                       .  . 

o  8c   to    ic. 

ic.  to  1.50. 

Simple  100  H  P  and  less   

1.5        "       2.5 

a.    "    5. 

Gas   20-50  HP  

2.0       ;     4.0 

4.    "    7. 

Gas   small                            •  . 

5            "80 

10     "  15. 

Steam   small                 

7.           "    12. 

12.      "  2O. 

By  small  engines  are  meant  those  not  over  15  to  20  HP,  such 
as  are  used  in  large  numbers  for  light  manufacturing  work. 
These  figures  are  of  course  only  approximate,  and  must  be  mod- 
ified by  the  cost  of  fuel  and  labor  in  any  particular  locality. 


THE   COMMERCIAL   PROBLEM.  5t 

They  take  no  account  of  the  efficiency  lost  between  the 
engine  and  its  work,  which  has  been  already  discussed  in 
Chapter  II.,  and  which  gives  motor  service  some  of  its  greatest 
•commercial  advantages. 

They  show  plainly,  however,  that  electrical  energy  delivered 
to  the  consumer  at  4  to  5  cents  per  kHowatt  hour  has  the 
commercial  advantage  in  small  work  of  all  kinds,  and  in  com- 
petition even  with  fairly  large  engines  used  at  light  load  or  in- 
termittently. In  addition  there  is,  in  favor  of  elecfricity,  the 
generally  considerable  saving  in  waste  power,  and  the  greater 
cleanliness  a<hd  convenience  of  the  motor.  At  equal  prices 
electric  power  will  pretty  effectively  keep  steam  out  of  all  new 
work,  but  the  cost  of  changing  from  one  motive  power  to  the 
other  demands  some  concessions  on  the  part  of  electricity. 

This  cost  of  change  is  rather  uncertain,  for  not  only  do  elec- 
tric motors  vary  very  widely  in  price,  owing  to  differences  in 
size,  speed,  and  construction,  but  the  net  value  of  engines  and 
boilers  replaced  may  vary  from  two-thirds  to  three-quarters 
of  their  cost  down  to  little  more  than  scrap. 

In  both  engines  and  motors  the  cost  of  the  smaller  sizes  is 
disproportionately  large,  owing  to  the  relatively  large  percent- 
age of  labor  in  their  construction.  Gas  engines  are  even  more 
expensive  than  a  steam  boiler  and  engine  in  ordinary  sizes. 
In  replacing  engines  by  motors,  the  selling  value  of  the  former, 
including  boilers,  if  steam  is  used,  may  be  anything,  say  from 
$10  to  $25  per  HP,  and  the  market  is  rather  uncertain  at  best. 
A  little  time  will  generally  effect  a  sale  on  tolerable  terms. 

The  following  table  gives  the  approximate  cost  of  electric 
motors  installed  and  ready  to  run,  based  on  motors  of  ordinary 
speeds  and  voltages,  with  the  usual  accessories  and  with  the 
simplest  sort  of  wiring.  No  useful  figures  can  be  given  on  the 
cost  of  special  installations  with  complex  wiring. 

From  this  it  appears  that  while  large  motors,  50  HP  and 
upward,  can  generally  be  counted  on  at  not  over  $25  per 
HP,  the  smaller  sizes  are  much  more  costly.  Below  twenty 
HP  the  net  cost  of  changing  from  steam  or  gas  engines  to 
motors  is  pretty  certain  to  be  $20  to  $30  per  HP.  Taking 
interest  and  depreciation  at  10  per  cent.,  the  annual  charge 
amounts  to  $2  or  $3  per  HP,  which  must  be  increased  to 
$5  or  $6  to  cover  maintenance  and  miscellaneous  expenses. 


562 


ELECTRIC    TRANSMISSION  OF  POWER. 


Hence,  for  steady  use  10  hours  per  day,  there  should  be 
charged  to  general  cost  about  0.2  cent  per  HP  hour,  which 
is  equivalent  to  perhaps  0.5  cent  for  intermittent  use. 


HP. 

COST. 

i 

$  100  to  $  125 

3 

175 

250 

5 

225 

275 

10 

325 

425 

15 

350 

450 

20 

450 

600 

25 

550 

700 

30 

650 

800 

40 

800 

1,000 

50 

900 

1,150 

75 

1,400 

1,700 

100 

1,900 

2,300 

In  changing  motive  power,  then,  electric  service  must  gen- 
erally be  cheaper  than  what  it  replaces  by  about  the  amounts 
mentioned. 

As  to  the  cost  of  furnishing  electric  power  figures  are  a  little 
deceptive,  since  from  place  to  place  the  conditions  vary.  It  is 
safe  to  allow  about  one  KW  at  the  station  for  one  HP  actually 
delivered  and  paid  for. 

Now  with  steam  for  a  motive  power  the  data  already  given 
for  mechanical  power  can  readily  be  reduced  to  kilowatt  hours, 
assuming  the  dynamos  to  have  as  usual  92  to  95  per  cent,  effi- 
ciency at  full  load.  But  a  steam  station  for  power  transmis- 
sion has  the  advantage  of  nearly  or  quite  continuous  running, 
thereby  reducing  general  expenses,  and  besides,  on  a  large 
scale,  the  load  can  be  kept  at  an  efficient  point  most  of  the 
time.  In  fact  in  large  railway  power  stations — the  only  steam- 
driven  stations  for  power  transmission  on  a  large  scale — the 
machines  can  be  worked  very  efficiently  most  of  the  time,  and 
power  can  be,  and  is,  very  cheaply  produced. 

Fig.  263  shows  graphically  the  approximate  variation  of  cost 
with  output  in  well-designed  power  stations,  the  figures  given 
being  based  on  $3  per  ton  for  coal  and  power  delivered  at 
the  station  bus  bars.  Anything  under  one  cent  per  KW  hour 
is  good  practice,  even  for  a  very  large  station.  Steam  is  not 
likely  to  be  often  used  as  a  motive  power  for  power  transmis- 
sion work,  except  in  working  a  very  cheap  coal  supply. 


THE   COMMERCIAL   PROBLEM. 


563 


Dr.  Emery  has  worked  out  at  considerable  length  the  prob- 
lem of  the  cost  of  steam  power  on  a  very  large  scale  and  with 
the  most  economical  modern  machinery.  He  assumed  a 
20,000  HP  plant,  worked  24  hours  per  day,  on  a  variable  load 
averaging  12,760  HP,  63.8  per  cent,  of  the  maximum.  This 


500  1000  1500 

Capacity  of  station  in  Kilowatts 

FIG.  263. 


2000 


load  factor  is  judiciously  estimated  and  could  certainly  be 
realized  in  a  plant  of  such  size,  employed  in  the  general  dis- 
tribution of  power.  Taking  coal  at  one  mill  per  pound,  $2.24 
per  long  ton,  and  entering  every  item  of  expense,  he  found  the 
total  cost  per  HP  per  year  to  be  $33.14.  If  the  plant  were 
established  at  the  mouth  of  the  coal  mine,  fuel  should  be 
obtained  at  not  over  one-third  the  above  cost.  This  advan- 
tage would  bring  the  cost  per  HP  per  year  down  to  $24.89. 


564  ELECTRIC   TRANSMISSION  OF  POWER. 

Taking  now  15,000  KW  in  dynamo  capacity  in  large  direct 
coupled  units,  say  eight  in  number,  the  electrical  plant  would 
cost,  installed  with  all  needful  accessories  and  ready  to  run, 
$375,000.  Taking  interest,  taxes  and  depreciation  together 
at  10  per  cent.,  which  is  enough,  since  a  3  per  cent,  sinking 
fund  would  amply  allow  for  depreciation;  allowing  $15,000  per 
year  for  additional  labor  and  superintendence  and  $10,000  more 
for  maintenance  and  miscellaneous  expenses,  brings  the  total 
annual  charge  for  the  electrical  machinery  to  $62,500.  Add- 
ing this  to  the  steam  power  item  and  reducing  the  whole  to 
cost  per  KW  hour,  assuming  94  per  cent,  average  dynamo 
efficiency,  the  total  cost  per  KW  hour  delivered  at  the  station 
switchboard  becomes  0.482  cent.  Working,  then,  on  an 
immense  scale  from  cheap  coal,  it  is  safe  to  say  that  half  a 
cent  per  KW  hour  will  deliver  the  energy  to  the  bus  bars. 

The  next  step  is  the  cost  of  delivering  it  to  the  customer. 
This  varies  so  greatly,  according  to  circumstances,  that  an 
average  is  very  hard  to  strike.  A  plant  such  as  we  are  con- 
sidering will  usually  be  installed  only  when  the  radius  of  dis- 
tribution is  fairly  long.  Taking  the  transmission  proper  as 
20  miles,  the  line  and  right  of  way,  using  10,000  volts,  may 
be  taken  as  about  $25  per  KW;  the  raising  and  reducing 
transformers  with  sub-station  and  equipment  would  cost 
another $25  per  KW,  and  the  distributing  circuits,  with  a  good 
proportion  of  large  units,  about  $5  per  KW  additional.  The 
distributing  system  for  15,000  KW  would  then  cost  abo 
$975,000.  Figuring  interest  and  depreciation  roundly  at  10 
per  cent,  the  annual  charge  is  $97,500.  Add  now  $15,000  for 
labor  in  sub-station  and  distributing  system,  $10,000  for  general 
administrative  expense,  and  5  per  cent,  for  maintenance  and 
miscellaneous  expenses,  and  we  reach  a  total  annual  charge  for 
distribution  of  $171,250.  The  average  output  being  almost 
exactly  9,000  KW,  the  cost  of  distribution  per  KW  hour  is 
0.218  cent.  The  actual  cost  of  generating  and  distributing 
the  power  then  becomes  0.700  cent  per  KW  hour. 

This  is  probably  a  minimum  for  distribution  of  power  from 
coal  mines.  It  supposes  a  very  large  plant  installed  for  cash 
and  operated  for  profit.  It  makes  no  allowance  for  the  float- 
ing of  bonds  at  60  to  80  cents  on  the  dollar,  the  operations  of 
a  construction  company,  the  purchase  of  coal  from  the  direct- 


THE   COMMERCIAL   PROBLEM.  565 

ors,  the  payment  of  big  salaries  to  the  promoters,  or  any 
of  the  allied  devices  well-known  in  financial  circles. 

Under  fayorable  circumstances  at  least  an  equivalent  result 
can  be  reached  with  hydraulic  power. 

These  figures  mean  that  power  could  be  sold  at  an  average 
of  i  cent  per  KW  hour  at  a  good  profit,  aggregating  for  the 
plant  in  question  nearly  a  quarter  of  a  million  dollars  per  year. 

Only  the  largest  plants,  skillfully  handled,  can  approach 
such  figures  for  cost  of  power  as  have  just  been  given. 

It  should  be  possible,  however,  to  bring  the  cost  of  distri- 
bution per  fcW  hour  in  a  well-designed  transmission  plant  of 
1000  HP  or  more  down  to  less  than  0.5  cent  per  KW  hour. 
Less  than  this  may  indeed  be  found  in  practice,  while  figures 
approaching  0.25  cent  may  be  found  in  good  central  station 
working. 

The  cost  of  producing  power  in  steam-driven  plants  of  vari- 
ous sizes  has  already  been  given;  that  in  water  power  plants  is 
far  less  definite,  but  on  the  whole  lower.  In  some  hydraulic 
plants  where  development  has  been  costly,  the  cost  of  water 
power  rises  to  $20  or  $25  per  net  HP  year,  while  on  the 
other  hand  water  power  has  been  leased  at  the  canal  for  as 
little  as  $5  per  year  per  hydraulic  HP  in  the  canal,  equivalent 
to  about  $6.50  per  available  HP  at  the  wheel  shaft.  The 
investment  per  effective  HP  at  the  wheels  ranges  from  nearly 
$150  to  as  low  as  $30  or  $40.  This  includes  both  the  hydraulic 
rights  and  work,  and  the  wheels  themselves. 

A  typical  estimate  for  a  water  power  plant  under  fairly  fav- 
orable conditions,  derived  from  actual  practice,  runs  about  as 
follows,  for  a  1000  HP  plant: 

Hydraulic  works,             .         ..        .         .         .  $40,000 

Wheels  and  fittings,         .        V        .         .         .  12,500 

Power  station,         .         .         .         .         *         .  2,500 

Pole  line,  8  miles,          .         .         .         .         j  4,000 

Transmission  circuit,      .                  .         .         .  15,000 

Dynamos  and  equipment,   700  KW,       . ~"      .  20,000 

Transformers,   750  KW,         ....  10,000 

Distributing  lines,            .....  15,000 

Miscellaneous,         ......  5,ooo 

Total, $124,000 


566  ELECTRIC   TRANSMISSION  OF  POWER. 

Operating  expense: 

Interest  and  depreciation,  10  per  cent., 
Attendance  at  plant,  .... 
Linemen  and  team,  .... 

Office  expense, 

Rent,  taxes,  and  incidentals, 
Maintenance  and  supplies, 

Total,      .         .         .         .         .         .  $26,900 

The  full  capacity  of  the  plant  is  about  700  KW.  Supposing 
the  plant  to  be  worked  somewhere  near  its  capacity  at  maxi- 
mum load,  and  to  be  in  operation  on  a  mixed  load  24  hours 
per  day,  we  may  estimate  the  daily  output  about  as  follows: 

KW  KVVH 

9  hours  @  500  .  .  .         .         .         .  ^       .  4,500 

5  "      "  250  .  .  .         .         .         .         .   1,250 

3     "      "  100  .  '  .  . 300 

6  "      "    50  .  .   .  .         .         .         ...      300 

Total,         .         .         .         .         .      .   .  6,350 

This  should  be  taken  for  300  days  in  the  year.  The  other 
65  days,  Sundays,  holidays  and  occasional  periods  of  unusually 
small  motor  loads,  it  is  not  safe  to  count  on  more  than  1,000 
KW  hours  per  day.  Taking  account  of  stock  we  have  for  the  year 

1,970,000  KWH, 

and  the  net  cost  per  kilowatt  hour  becomes  1.36  cent.  It  is 
worth  noting  that  the  distribution  of  power  for  the  day  is  taken 
from  a  transmission  plant  in  actual  operation. 

Of  the  above  total  cost  0.47  cent  is  chargeable  to  distribution 
expenses  and  0.89  to  power  production.  Doubling  the  cost  of 
the  hydraulic  works  would  raise  the  generating  cost  to  1. 10  cent 
and  the  total  cost  to  1.57. 

It  is  evident  in  this  case  that  power  could  be  sold  at  2  cents 
net  per  HPH  with  a  good  profit,  assuming  the  smaller  total 
cost,  and  at  2.5  cents,  even  with  the  greater  hydraulic  cost. 
Even  if  the  total  investment  were  as  great  as  $250,000,  the 
plant  would  pay  fairly  well  at  3  cents  per  HPH. 

The  fact  is,  hydraulic  transmission  plants  generally  will  pay 
well  if  a  good  load  can  be  obtained.  The  above  example  does 
not  show  a  specially  cheap  plant  nor  a  remarkable  load  factor. 


THE   COMMERCIAL  PROBLEM.  567 

In  really  favorable  cases  the  cost  of  power  distributed  will 
not  exceed  i  cent  per  HP  hour,  and  in  comparatively  few 
plants  will  it  rise  to  2  cents,  unless  the  market  for  power  is 
grossly  overestimated. 

This  is  one  of  the  commonest  troubles  with  plants  that  do 
not  pay  well.  A  costly  hydraulic  development  is  undertaken, 
resulting  in  rendering  available  several  times  as  much  power 
as  can  be  utilized;  a  portion  of  this  is  then  transmitted  and 
sold,  but  the  plant  is  burdened  with  heavy  initial  expense,  and 
struggles  along  as  best  it  can.  It  is  not  safe  to  count  on  the 
stimulation  of  industrial  growth  by  cheap  power  unless  the 
cost  of  producing  power  is  so  small  that  the  plant  will  pay 
tolerably  well  on  the  existing  market. 

A  careful  canvass  for  power  is  a  necessary  part  of  the  pre- 
liminary work  for  a  power  transmission,  and  the  more  com- 
plete it  can  be  made  the  better.  Reference  to  the  table  of 
p.  560  shows  that,  at  a  selling  rate  of  2  to  3  cents  per  HP 
hour,  the  cost  of  power  can  be  reduced  for  all  small  consumers 
and  a  good  many  rather  large  ones.  If  the  cost  of  coal  is 
high,  $5  per  ton  or  more,  nearly  all  consumers  will  save  by 
using  electric  power,  while  with  favorable  hydraulic  conditions 
money  can  be  saved  by  transmission  even  when  replacing  very 
cheap  steam  power. 

Take,  for  example,  a  large  manufacturing  plant  requiring 
1,000  HP  steadily,  12  hours  a  day.  At  a  distance  of,  say  8 
miles,  is  a  hydraulic  power  that  can  give,  say  1,200  HP,  and  can 
be  purchased  and  developed  for  $100,000.  The  cost  of  gener- 
ating and  transmitting  power  will  be  about  as  follows: 

Hydraulic  work,        .....  $100,000 

Wheels  and  fittings,           ....  15,000 

Power  house^ 3,ooo 

Pole  line, 4,000 

Dynamos,  1,000  KW,         ....  25,000 

Transmission  circuit,         ....  15,000 

Motors  and  equipment,     ....  25,000 

Miscellaneous, 10,000 


Total,         •  «      .         .         .         .         .        $197,000 
and  the  operating  expenses  would  be  about  as  follows: 


568  ELECTRIC   TRANSMISSION  OF  POWER. 

Interest  and  depreciation,         .         .         .  $19,700 

Attendance  at  plant,         ....  2,500 

"             "  motors,      ....  i, 800 

Other  labor,      ......  1,000 

Maintenance,  supplies,  etc.,      .         .         .  5,ooo 


Total,  .         $30,000 

This  would  furnish,  taking  the  working  year  as  308  days, 
3,696,000  HP  hours  at  a  cost  of  0.81  cent  per  HP  hour.  With 
a  low  cost  of  hydraulic  development  and  a  short  line,  say  not 
over  three  miles,  the  above  figures  for  cost  could  be  brought 
down  to  about  $130,000.  Now,  allowing  5  per  cent,  for 
interest  and  setting  aside  3  per  cent,  for  sinking  fund,  which 
allows  for  complete  replacement  in  less  than  20  years,  we 
may  figure  the  annual  cost  of  power  again  thus: 

Interest  and  sinking  fund,  .  .  .  $10,400 

Attendance  at  plant,          .  .  .  2,500 

"          "  motors,       .  .  .  .  i, 800 

Maintenance  and  incidentals,  .  .  .  S,ooo 


Total,         .         .         .         .         .         .         $19,700 

This  is  $19.70  per  HP  year,  or  0.53  cent. per  HP  hour.  This 
is  certainly  cheaper  than  power  can  be  generated  by  steam, 
even  with  coal  at  $1.50  or  so  per  ton,  provided  proper 
account  be  taken  of  interest,  depreciation  and  repairs.  As  a 
matter  of  fact,  the  cost  just  given  has  been  reached,  in  practice, 
in  transmission  work  at  moderate  distances.  On  a  larger  scale 
slightly  better  results  can  be  attained.  These  figures  take  no 
account  of  the  saving  in  actual  power  obtained  by  distributed 
motors,  always  an  important  matter  in  organizing  a  transmis- 
sion for  manufacturing  purposes.  This  can  generally  be 
counted  on  to  make  it  possible  to  replace  1,000  HP  in  a  steam 
engine  by  not  over  750  HP  in  electric  motors,  with  a  corre- 
sponding reduction  in  the  aggregate  yearly  cost  of  power. 

Speaking  of  costs  in  a  general  way,  dynamos  and  their 
equipment  may  safely  be  taken  at  $25  per  kilowatt,  raising 
and  reducing  transformers  at  from  $8  to  $12  per  KW,  line 
erected  at  from  $ro  to  $30  per  KW,  water  wheels  at  $10  to 


THE   COMMERCIAL  PROBLEM.  569 

$20  per  HP,  and  steam  plant,  when  used,  at  from  $50  to  $6c> 
per  net  HP. 

The  line  is  always  a  rather  uncertain  item,  on  account  of  its 
variations  in  cost  at  different  distances,  and  in  meeting  local 
conditions  of  distribution.  The  pole  line  itself  will  cost  from 
$250  to  $500  per  mile,  according  to  circumstances,  but  the 
copper  must  be  figured  separately,  as  already  explained. 

No  account  is  here  taken  of  freaks  in  design — dynamos  of 
special  design  for  peculiar  speeds  or  voltages,  extraordinary 
line  voltages,  unusual  frequencies,  or  eccentric  methods  of 
distribution  like  the  wholesale  use  of  rotary  converters  and 
storage  batteries.  The  figures  are  intended  to  represent 
ordinary  good  practice  as  it  exists  to-day. 

One  of  the  nicest  points  in  operating  a  transmission  plant  is 
the  proper  adjustment  of  the  price  of  power  to  the  existing 
market.  It  is  no  easy  matter  to  strike  the  point  between  the 
cost  of  other  power  and  the  cost  of  generating  and  distribut- 
ing electric  power,  which  will  give  the  maximum  net  profit. 
In  general  it  is  best  to  work  entirely  on  a  meter  basis,  for 
the  customer  then  pays  simply  for  what  he  uses,  and  the  sta- 
tion manager  knows  the  exact  distribution  of  his  output. 

The  generating  station  or  the  sub-station  should  be  equipped 
with  a  recording  wattmeter  that  will  show  the  actual  output, 
and  from  this  measurement  much  valuable  information  can  be 
obtained. 

Knowing  the  investment  and  the  approximate  operating 
expense  it  is  easy  to  figure,  as  we  have  just  done,  the  total 
cost  of  delivering  energy  per  KW  power  at  various  outputs. 
This  is  the  basis  of  operations.  The  next  thing  is  to  estimate 
as  closely  as  possible  the  average  local  cost  of  power  in 
units  of  various  sizes.  These  two  quantities  form  the 
possible  limits  on  selling  price.  One  must  keep  far  enough 
above  the  first  to  insure  a  good  profit,  and  enough  below  the 
second  to  capture  the  business.  It  is  convenient  to  plot  these 
data  as  in  Fig.  264,  which  is  based  on  the  table  of  p.  560,  and 
the  plant  discussed  on  p.  566.  Curve  T  shows  the  effect  of  change 
in  the  annual  output  on  the  net  cost  per  KWH.  Curve  2 
shows  the  approximate  existing  cost  of  steam  or  other  power, 
the  points  from  which  the  curve  was  drawn  being  shown  by 
crosses.  Curve  3  shows  the  same  for  intermittent  loads,  the 


57° 


ELECTRIC    TRANSMISSION  OF  POWER. 


points  being  indicated  by  circles.  It  is  evident  that  for 
yearly  outputs  less  than  1,000,000  KW  hours  the  plant  would 
be  in  bad  shape  to  get  business.  At  2,000,000  KWH  good 
profits  are  in  sight,  while  at  3,000,000  the  electric  plant  can 
meet  all  cases  at  a  profit. 

At  the  given  output  of  1,970,000  KWH  it  would   be  pos- 
sible to   charge  2    cents   per   KWH   as   a  minimum   without 


H.P 


500       1000       1500      2000      2500      3000       3500      4000 
ANNUAL  OUTPUT  THOUSANDS  OF  K.W.  HOURS 


FIG.  264. 

losing  much  business,  while  all  the  smaller  customers  could 
gain  by  changing  to  electric  power  at  3,  4  or  5  cents  per 
KW  hour. 

When  a  few  consumers  are  generating  power  at  an  unusually 
low  figure  there  is  always  the  temptation  to  obtain  them  at  a 
special  cut  rate.  As  a  rule  this  is  bad  policy  unless  they 


THE   COMMERCIAL    PROBLEM.  571 

are  desirable  for  some  particular  reason  aside  from  increase 
of  output,  for  the  moral  effect  of  special  low-price  contracts  is 
always  bad,  and  in  the  long  run  it  is  best  to  make  standard 
rates  and  to  adhere  to  them. 

The  best  prices  can  always  of  course  be  obtained  from  small 
consumers,  and  these  are  also  specially  desirable  in  that  they 
tend  to  keep  a  uniform  load  on  the  system.  Not  only  do 
50  10  HP  motors  yield  several  times  as  much  revenue  as  one 
500  HP  motor,  but  they  will  call  for  power  very  steadily  all 
day  long  and  keep  the  regulation  excellent,  while  the  large 
motor  may  be  off  and  on  in  the  most  exasperating  way  and 
cause  great  annoyance  at  the  time  of  the  "lap  load,"  when 
lights  and  motors  are  all  in  use.  Large  motors  running  inter- 
mittently are  disadvantageous,  for  they  do  not  greatly  increase 
the  aggregate  station  output  and  pay  relatively  little. 

In  general  the  best  schedule  of  prices  can  be  made  up  by 
starting  with  a  rate  arranged  to  get  all  the  powers  below,  say 
4  or  5  HP,  and  then  for  larger  powers  arranging  a  set  of  dis- 
counts from  this  initial  rate.  These  discounts,  however, 
should  be  based  not  directly  upon  the  size  of  the  motors,  but 
•on  the  monthly  KW  hours  recorded  against  them.  In  one 
respect  charging  by  wattmeter  alone  is  at  rather  a  disadvan- 
tage. A  large  motor  running  at  variable  load,  and  much  of 
the  time  at  light  load,  is  far  less  desirable  as  a  station  load 
than  a  small  and  steadily  running  motor  using  the  same  num- 
ber of  KW  hours  monthly.  The  former  demands  far  greater 
station  capacity  for  the  same  earning  power,  and  also  inflicts 
a  bad  power  factor  upon  the  system  at  times  of  light  load  if 
the  distribution  is  by  alternating  current.  It  is  not  easy  to 
avoid  this  difficulty,  although  various  devices  to  that  end  have 
been  introduced,  In  one  large  plant  recording  ammeters  are 
installed  for  each  motor,  and  the  largest  demand  for  current 
lasting  two  minutes  or  more  during  a  given  month  is  made  a 
factor  in  determining  the  price  paid  for  that  month's  supply 
of  power,  so  that  large  demands  for  station  capacity  must 
in  part  be  paid  for  by  the  consumer. 

Another  device  for  the  same  purpose  is  a  combination  of  the 
flat-rate  and  meter  methods  of  charging.  A  fixed  monthly 
charge  per  horse  power  of  the  motor  connected  is  made,  and  in 
addition' the  consumer  pays  for  his  energy  by  wattmeter,  of 


57 2  ELECTRIC    TRANSMISSION  OF  POWER. 

course  at  a  somewhat  lower  rate  than  in  using  the  meter  alone. 
A  rough  illustration  of  the  effect  is  as  follows.  Suppose  a  flat 
charge  of  $i  per  month  per  HP  of  the  motor  installed  and  a 
meter  rate  of  3  cents  per  KWH.  One  customer  has  a  10  HP 
motor  worked  steadily  at  full  load  10  hours  per  day  for  30  days. 
Another  has  a  50  HP  motor  which  runs  at  full  load  for  2 
hours  per  day.  Each  may,  for  example,  use  3000  KWH  per 
month,  and  pay  by  meter  $90  therefor;  but  the  former  pays  a 
flat  charge  of  $10,  the  latter  one  of  $50,  so  that  the  monthly 
bill  is  in  the  former  case  $100,  in  the  latter  $140.  The  extra 
$40  may  be  regarded  as  the  payment  of  rent  for  station 
capacity  and  capacity  of  lines  and  transformers,  to  be  held 
subject  to  the  customer's  call  at  any  time.  It  is,  in  fact,  a  real 
expense  to  the  station,  and  may  be  considered  a  proper  sub- 
ject for  a  charge  to  the  customer. 

Charging  by  a  recording  ammeter  instead  of  a  wattmeter 
will  reach  the  user  of  motors  that  injure  the  power  factor  of 
the  system,  and,  combined  with  the  flat  rate  just  mentioned, 
would  probably  give  a  really  fairer  system  of  payment  for  the 
customer's  demand  upon  the  station  than  either  of  the  schemes 
just  described. 

Methods  of  selling  and  charging,  however,  must  be  modified 
to  suit  local  conditions  and  customs.  Each  community  hai 
peculiarities  of  its  own  that  must  be  studied  and  reached. 
Sometimes  a  flat  rate,  objectionable  as  it  often  is,  will  secure 
a  more  remunerative  business  than  any  system  of  metering, 
while  elsewhere  a  meter  system,  however  intricate,  may  work 
better  than  a  flat  rate.  As  a  rule,  however,  metering  is  the 
best  method  of  charging  for  all  parties. 

A  water  power  transmission  plant  has  the  peculiarity,  when, 
as  usual,  the  water  is  owned  outright,  of  showing  a  nearly 
constant  operating  expense,  irrespective  of  output.  Hence 
after  the  receipts  exceed  this  expense  all  additional  load,  at 
any  price,  means  profit.  But  it  means  profit  precisely  in  pro- 
portion to  its  price,  so  that  taking  on  large  consumers  at  a 
very  low  price  is  usually  bad  policy,  it  being  better  to 
encourage  small  consumers  by  giving  what  is  to  them  a  very 
reasonable  figure. 

In  the  case  in  hand,  it  would  probably  be  worth  while  to 
start  as  low  as  5  cents  per  KW  hour  on  a  monthly  consump- 


THE    COMMERCIAL   PROBLEM.  57 J 

tion  up  to  1000  KW  hours.  This  corresponds  nearly  to  4  HP 
used  continuously  9  hours  per  day.  For  100  HP  so  used  it 
should  be  possible  to  get  2  cents  per  KW  hour,  so  for  a  con- 
sumption of  25,000  KW  hours  per  month  the  discount  fixes 
itself  at  60  per  cent.  For  intermediate  points  the  proper  dis- 
counts are  fixed  in  a  similar  manner.  A  uniform  discount  of 
say  10  per  cent,  from  bills  for  payment  before  a  fixed  date  is 
generally  good  policy.  The  rates  of  discount  should  be  set 
bearing  in  mind  the  distribution  of  business  with  respect  to  the 
size  of  units,  the  intent  being  to  get  all  the  consumers  of 
moderate  power.  Above  a  fixed  amount  the  special  contract, 
generally  undesirable,  may  sometimes  become  useful. 

After  the  maximum  output  comes  near  to  the  capacity  of 
the  plant,  the  total  yearly  output  for  the  given  plant  is  diffi- 
cult to  increase.  Hence  it  is  desirable  persistently  to  culti- 
vate the  use  of  power  at  such  times  as  will  not  increase  the 
maximum  load.  This  can  best  be  done  by  offering  liberal  dis- 
counts for  power  used  only  between  say  8  P.  M.  and  8  A.  M. 
There  is  at  best  rather  a  small  amount  of  this,  and  it  is  all 
worth  getting  even  at  a  low  rate. 

In  stations  using  rented  water  power  at  a  fixed  price  per 
HP,  or  "employing  steam,  the  operating  expense  is  of  course 
variable,  and  this  variation  will  influence  greatly  the  adjust- 
ment of  prices,  although  the  general  principles  are  unchanged. 

Experience  has  now  shown  that  electric  power  transmission 
may  generally  be  made  a  profitable  enterprise. 

If  a  transmission  is  planned  and  executed  on  sound  business 
principles  and  with  ordinary  forethought,  it  is  well-nigh  certain 
to  be  a  permanent  and  profitable  investment. 

Failure  is  generally  chargeable  to  attempts  to  work  with 
altogether  insufficient  capital,  leading  to  ruinous  actual  rates 
of  interest;  the  purchase  of  material  at  extortionate  prices 
due  to  various  forms  of  credit;  and  huge  commissions  to 
promoters. 

Organized  in  such  wise  almost  any  enterprise  becomes 
merely  speculative,  and  its  failure  should  produce  neither 
surprise  nor  sympathy,  for  such  a  course  is  the  broad  highway 
that  leads  straight  into  the  ever  ready  clutches  of  a  receiver. 
Honesty  is  the  best  policy  in  power  transmission,  as  else- 
where. 


CHAPTER   XVI. 

THE    MEASUREMENT    OF    ELECTRICAL    ENERGY. 

THE  basic  fact  regarding  the  measurement  of  electrical 
power  is  the  stress  between  a  magnetic  field  and  a  coil  carry- 
ing a  current.  Obviously  such  a  coil  produces  of  itself  a 
magnetic  field,  but  it  is  the  proportionality  of  this  field  to  the 
current  rather  than  its  mere  existence  that  gives  it  importance 
in  measuring  instruments. 

The  fundamental  measurements  which  have  to  be  made  in 
ordinary  practical  engineering  are  three— current,  electro- 
motive force,  and  electrical  energy,  which  is  their  co-directed 
product.  In  continuous  current  work,  while  mere  readings 
of  the  first  two  give  the  energy  as  their  numerical  product,  it  is 
generally  desirable  to  have  instruments  which  measure  energy 
directly  and  which  integrate  a  varying  output  continuously, 
so  that  one  may  at  all  times  keep  track  of  the  output  of  the 
station,  a  single  circuit,  or  the  energy  supplied  to  a  single 
customer.  In  alternating  current  work  a  wattmeter  is  doubly 
necessary,  first  because  the  product  of  volts  and  amperes  does 
not  give  the  real  energy,  but  the  apparent  energy,  as  has 
already  been  explained;  and,  second,  because  the  true  energy 
divided  by  the  apparent  energy  equals  the  power  factor,  which 
should  be  looked  after  very  carefully  in  an  alternating  station. 

Any  effect  of  electric  current  which  is  proportional  to  or 
simply  related  to  that  current  may  obviously  be  used  for  its 
measurement,  and  in  laboratory  measurements  instruments 
based  on  almost  every  imaginable  property  of  electric  current 
have  been  used  with  more  or  less  success.  But  for  everyday, 
practical  purposes  instruments  must  possess  qualities  not  so 
needful  in  the  laboratory,  so  that  the  possible  types  of  measur- 
ing instrument  have  simmered  down  to  a  very  few,  with  re- 
spect to  the  principles  concerned. 

So  far  as  continuous  currents  are  involved,  nearly  all 
practical  instruments  are  electro-magnetic,  as  has  already 

574 


THE   MEASUREMENT  OF  ELECTRICAL   ENERGY.      575 

been  indicated— almost  the  sole  exception  being  the  Edison 
chemical  meter,  which  need  not  here  be  described,  since  it  is 
passing  rapidly  out  of  use. 

The  simplest  electrical  measuring  instrument  is  the 
ammeter •,  designed  for  the  practical  measurement  of  ceirrent 
strength.  In  its  commonest  forms,  as  used  for  continuous  cur- 
rent, it  consists  of  a  fixed  coil  of  wire  carrying  the  current  to  be 
measured  and  a  pivoted  magnetic  core,  to  which  is  attached  a 
pointer  sweeping  over  a  fixed  scale.  The  force  on  this  core 
varies  with  the  current,  and  is  resisted  by  some  opposing  force 
that  brings  the  pointer  into  a  new  point  of  equilibrium  for  each 
value  of  the  current.  Sometimes  this  opposing  force  is  the 
magnetic  field  of  the  earth,  as  in  the  ordinary  laboratory 
galvanometer,  but  in  practical  instruments  it  is  generally 
gravity,  a  spring,  or  a  relatively  powerful  permanent  magnet. 

Most  of  the  numerous  varieties  of  ammeter  have  been 
produced  in  the  effort  to  secure  a  permanent  and  constant 
controlling  force,  and  uniformity  of  scale;  that  is,  -such  an 
arrangement  of  parts  as  will  make  the  angular  deflection  of  the 
pointer  directly  proportional  to  the  amperes  flowing  through 
the  coil.  The  result  has  been  all  sorts  of  curious  arrange- 
ments of  the  coils  and  the  moving  armature  with  respect  to 
each  other,  and  the  upshot  of  the  matter  generally  is  that  the 
scale  has  to  be  hand  calibrated  for  each  instrument,  the 
divisions  of  the  scale  being  fairly  uniform  through  the  parts 
of  the  scale  most  often  used,  but  varying  somewhat  near  its 
ends.  Gravity  is  far  and  away  the  most  reliable  controlling 
force,  but  it  is  also  highly  inconvenient  in  instruments 
intended  for  portable  use  or  for  a  wide  range  of  action  while 
still  preserving  small  inertia  in  the  moving  parts,  so  that  springs 
or  permanent  magnetic  fields  form  the  main  reliance  in  practice. 
In  some  admirable  instruments  the  well-known  principle  of  the 
D'Arsonval  galvanometer  is  employed.  In  this  instrument,  of 
which  one  type  is  shown  in  Fig.  265,  a  light  movable  coil  is 
suspended  between  the  poles  of  a  very  powerful  permanent 
magnet,  shown  in  the  cut  as  built  up  in  circular  form.  Current 
traversing  the  coil  through  the  suspension  wires  sets  up  a  field, 
which,  reacting  with  the  magnet,  produces  a  powerful  deflect- 
ing force  on  the  coil,  controlled  by  the  torsion  suspension.  In 
commercial  instruments  the  suspension  is  replaced  by  jeweled 


576 


ELECTRIC   TRANSMISSION   OF  POWER. 


bearings,  and  the  current  is  led  in  through  the  controlling  hair 
springs  or  by  very  flexible  leads.  The  resulting  instrument  is 
very  sensitive  and  accurate  for  the  measurement  of  small 
currents  or  known  fractions  shunted  from  larger  ones.  The 


FIG.  265. 

famous  Weston   instruments,  together  with  others   less  well 
known,  are  constructed  along  this  general  line. 

The  sources  of  error  even  in  the  best  commercial  ammeters 
are  many.  Permanent  magnets  and  springs  do  not  always 
hold  their  strength  precisely,  jeweled  bearings  wear,  and 
break  if  the  instruments  are  roughly  handled,  pointers  get 
bent,  dust  sometimes  gets  in,  and  on  these  accidental  errors 
are  superposed  those  due  to  errors  in  scale  and  calibration. 


THE   MEASUREMENT  OF  ELECTRICAL   ENERGY.      577 

Nevertheless  the  best  station  and  portable  ammeters  possess 
and  maintain  a  very  commendable  degree  of  accuracy.  When 
carefully  handled  and  used  well  within  their  working  range 
they  can  be  trusted  to  within  about  one  or  two  per  cent.  If 
of  the  highest  grade  and  frequently  verified  they  can  be  relied 
on  in  the  best  part  of  the  scale  down  to,  say,  half  the  above 
amount,  and  under  circumstances  exceptionally  favorable  will 
do  even  a  little  better.  In  laboratory  work,  where  they  are 
merely  used  as  working  instruments  and  often  checked,  it  is 
possible  to  nurse  them  into  still  higher  accuracy,  but  one 
cannot  depend  upon  it  for  long  at  a  time  under  commercial 
conditions.  For  relative  measurements  only,  made  within  a 
short  time,  high  grade  ammeters  are  very  accurate,  but  the 
hints  already  given  should  make  it  clear  that  when  in  regular 
use  one  must  not  expect  to  use  them  for  absolute  measure- 
ments with  a  great  degree  of  precision.  The  cheaper  class  of 
instruments  is  likely  to  show  double  the  errors  just  noted. 

For  the  measurement  of  alternating  currents  only  a  few  of 
the  types  of  ammeter  used  for  continuous  current  are 
applicable.  Hysteresis  in  the  iron  parts  and  reactance  in  the 
coils  are  likely  to  incapacitate  them,  but  some  of  the  forms 
can  readily  be  modified  to  give  good  results,  and  certain 
others  are  specially  suited  to  alternating  currents.  In  this 
work  a  new  class,  having  a  fixed  field  coil  reacting  on  a  closed 
circuited  armature  coil  capable  of  rotation,  and  spring  con- 
trolled, has  been  made  useful;  and  on  account  of  their 
extremely  small  reactance  hot  wire  instruments  have  retained 
some  measure  of  their  one-time  popularity.  In  this  class  of 
instruments  the  current  is  passed  through  a  fine  suspended 
wire  of  rather  large  resistance,  which  is  thereby  heated  and 
expands,  carrying  with  it  the  pointer  to  which  it  is  attached, 
usually  by  means  of  multiplying  gear.  Such  instruments 
require  correction  for  the  temperature  of  the  air,  but  are 
capable  of  very  good  accuracy  if  carefully  handled.  They 
are  "dead  beat,"  /.  ^.,  the  pointer  comes  to  rest  without 
oscillation,  a  very  useful  property,  which  is  secured  to  a  cer- 
tain extent  in  most  instruments  by  various  damping  devices. 
Instruments  having  a  powerful  permanent  magnet  often  are 
supplied  with  a  copper  damping  vane,  which  checks  oscillations 
by  virtue  of  the  eddy  currents  stirred  up  in  it  by  the  magnet, 


57 8  ELECTRIC   TRANSMISSION  OF  POWER. 

and  sometimes  air  vanes  in  a  close-fitting  recess  or  light 
mechanical  stops,  which  can  be  brought  up  against  the  moving 
parts,  are  used  for  this  purpose. 

Voltmeters  for  measuring  the  electromotive  force  are  in 
all  general  points  constructed  precisely  like  ammeters,  save 
that  the  working  coil,  whether  fixed  or  movable,  is  wound  with 
very  fine  wire  in  many  turns,  so  as  to  be  adapted  to  work  with 
very  small  currents,  and  usually  has  in  series  with  it  a  resist- 
ance of  several  thousand  ohms.  Voltmeters  are  in  fact 
ammeters  having  so  much  resistance  permanently  in  circuit 
that  the  current  which  flows  through  them  is  substantially 
proportional  to  the  voltage  across  the  points  to  which  the 
instrument  is  connected,  irrespective  of  other  resistances 
which  may  casually  be  in  circuit.  They  are  somewhat  more 
difficult  to  construct  than  ordinary  ammeters,  owing  to  the  fine 
wire  windings  and  the  high  resistance,  and  are  generally  rather 
more  expensive. 

They  are  capable  of  just  about  the  same  degree  of  precision 
as  ammeters,  being  subject  to  about  the  same  sources  of  error. 
When  used  for  alternating  current  the  large  auxiliary  resist- 
ance is  wound  non-inductively,  and  the  working  coil  is  pro- 
portioned for  as  low  reactance  as  may  be  possible  with  the 
required  sensitiveness.  For  measuring  very  high  alternating 
voltages  a  "potential  transformer,"  wound  with  an  accurately 
known  ratio  of  transformation,  receives  the  high  pressure 
current  and  delivers  it  to  the  voltmeter  at  a  more  reasonable 
voltage.  In  dealing  with  continuous  currents  the  problem  is 
more  difficult.  Sometimes  a  very  sensitive  voltmeter  is 
provided  with  a  separate  high  resistance  box,  reducing  the 
scale  readings  to  one-tenth  of  their  real  value,  so  that  the 
instrument  is  used  with  a  constant  multiplier  of  10,  to  trans- 
form its  readings  to  the  corresponding  voltage.  This  is  a  use- 
ful device  for  obtaining  the  voltage  of  arc  circuits  and  the  like. 

In  default  of  high  voltage  instruments  a  rack  of  incandescent 
lamps  may  be  wired  in  series  and  voltmeter  readings  taken 
across  a  known  fraction  of  the  total  resistance  thus  inserted; 
25o-volt  lamps  in  sufficient  number  not  to  be  brought  up  to 
full  candle-power  are  convenient  for  this  purpose,  and  the 
voltmeter  should  be  of  so  high  resistance  that  its  presence  as  a. 
shuntaround  part  of  the  lamps  will  not  introduce  material  erro** 


THE   MEASUREMENT  OF  ELECTRICAL   ENERGY.      $19 

A  generating  station  should  be  liberally  equipped  with 
ammeters  and  voltmeters.  Besides  the  ordinary  switch-board 
instruments,  usually  an  ammeter  for  each  machine  and  each 
feeder,  it  is  desirable  to  have  several  spare  instruments  which 
can  be  temporarily  put  in  for  testing  purposes.  Station 
instruments  should  have  large,  clearly  divided  scales  and 
conspicuous  pointers,  so  that  the  readings  can  be  seen  at  a 
distance  from  the  switch-board.  The  large  illuminated  dial 
instruments  are  excellent  for  the  principal  circuits,  and  the 
main  station  voltmeters  may  well  be  of  similar  type. 

Voltmeters  are  ordinarily  not  numerous  in  a  station,  and 
are  usually  arranged  with  changeable  connections,  so  that 
they  may  be  plugged  in  on  any  circuit.  There  should,  how- 
ever, always  be  at  least  one  conspicuous  voltmeter  permanently 
connected  to  show  the  working  pressure  on  the  main  circuits. 
In  polyphase  work  this  should  be  capable  of  being  plugged  in 
on  each  phase,  although  it  is  preferable  to  have  a  voltmeter 
permanently  on  each  phase  in  large  transmission  work.  At 
least  two  other  voltmeters  should  be  available  for  connection 
to  such  circuits  as  may  be  desirable,  in  testing  circuits, 
parallelizing  machines,  and  the  like.  These  ought  to  be  small 
switch-board  instruments  of  the  highest  grade,  mounted  side 
by  side  to  enable  comparative  readings  to  be  readily  made. 
As  potential  transformers  for  high  voltage  are  decidedly 
costly,  a  simple  and  safe  arrangement  for  plugging  in  the 
primary  side  of  such  a  transformer  on  any  high  voltage  con- 
nection is  much  to  be  desired.  A  duplicate  or  spare  potential 
transformer  should  always  be  kept  in  stock,  since  it  is  most 
inconvenient  to  have  a  voltmeter  thrown  out  of  action.  In 
stations  having  high  voltage  generators  it  is  sometimes  practi- 
cable to  connect  for  the  voltmeters  around  a  single  fixed 
armature  coil  in  each  generator,  which  much  simplifies  the 
transforming  arrangements. 

Indicating  wattmeters  reading  the  output  directly  are  not  in 
by  any  means  as  general  use  as  ammeters  and  voltmeters,  but 
are  highly  desirable  in  portable  form  for  motor  and  lamp 
testing,  and  should  be  seen  upon  the  switch-board  far  oftener 
than  they  are.  These  instruments  follow  the  same  general 
line  of  design  as  ammeters  and  voltmeters,  but  are  provided 
with  two  working  coils  or  sets  of  coils.  One  takes  the  current 


580  ELECTRIC    TRANSMISSION  OF  POWER. 

of  the  line  on  which  the  output  is  to  be  measured  and  the 
other  is  a  voltmeter  coil  suspended  so  as  to  turn  in  the  field 
due  to  the  current  coil.  The  torque  produced  obviously 
depends  on  the  product  of  the  two  fields  due  to  the  coils 
respectively,  which  is  proportional  to  the  energy  delivered. 
If  the  two  fields  are  in  the  same  phase,  as  in  continuous  current 
practice,  or  at  times  of  unity  power  factor  in  alternating 
circuits,  the  numerical  product  of  the  two  field  strengths  is 
proportional  to  the  total  energy;  but  if  there  is  difference  of 
phase,  then  the  co-directed  components  of  the  two  fields  are 
porportional  to  the  energy.  The  controlling  and  damping 
forces  are  like  those  in  ammeters  and  voltmeters,  and  the 
wattmeters  differ  little  from  them  in  general  arrangement 
save  for  having  two  sets  of  terminals,  one  for  current  and  the 
other  for  potential,  and  in  the  graduation  of  the  scale. 

A  well-equipped  station  should  have  two  or  three  such 
instruments  in  portable  form,  one  for  the  testing  of  incandes- 
cent lamps  and  such  small  outputs,  and  others  capable  of 
taking  the  output  delivered  to  the  ordinary  sizes  of  motors 
and  recording  wattmeters.  It  should  also  have  a  set  of 
portable  ammeters  capable  of  reading  the  ordinary  range  of 
customers'  currents  without  getting  off  the  good  working 
portions  of  their  respective  scales.  For  instance,  if  one 
ammeter  will  read  with  good  accuracy  from  i  to  10  amperes, 
the  next  might  go  effectively  from  5  to  25  amperes,  and  the 
next  from  20  to  60. 

Of  portable  voltmeters  there  should  be  enough  to  measure 
accurately  the  voltages  used  for  the  distribution,  and  a  portable 
potential  transformer  to  enable  primary  voltages  to  be  dealt 
with  in  an  alternating  system.  It  is  desirable  to  have  a  pair 
of  exactly  similar  voltmeters  to  use  in  simultaneous  readings 
for  drop  and  to  check  each  other  and  the  station  instruments. 

Another  form  of  voltmeter  regarded-  by  the  author  as  a 
necessity  in  every  power  transmission  plant  is  a  recording 
instrument  keeping  a  continuous  permanent  record  of  the 
voltage  and  its  variations.  Such  records  are  shown  reduced  in 
Figs.  258  and  259.  The  Bristol  voltmeter  is  the  form  of 
instrument  generally  used.  It  is  merely  a  strongly  made 
voltmeter  with  a  long  pointer  carrying  a  pen,  and  swinging 
from  center  to  circumference  of  a  paper  disc  driven  by  a 


THE   MEASUREMENT  OF  ELECTRICAL   ENERGY.      581 

clock  and  ruled  in  circles  for  the  volts  and  radially  for  time. 
A  variable  resistance  permits  it  to  be  accurately  adjusted 
to  agree  with  a  standard  voltmeter,  and  when  carefully 
managed  it  is  quite  reliable.  As  a  check  on  the  operation  of 
the  station  and  for  reference  in  case  of  dispute  it  is  invaluable, 
since  it  shows  every  variation  of  voltage,  and  the  time  at  which 
it  occurred.  In  using  it  the  pen  should  be  kept  clean  and 
smooth  running,  bearing  just  heavily  enough  to  leave  a  sharp, 
thin  line,  and  the  clock  should  be  very  carefully  adjusted 
to  keep  correct  time.  The  chart  should  be  changed  at 
the  same  time  each  day  and  put  on  so  as  to  record  the  correct 
time. 

Recording  .ammeters  and  steam  gauges  are  made  upon  the 
same  principle,  but  for  power  transmission  plants  the  volt- 
meter is  the  important  instrument.  Installed  in  the  generat- 
ing station  it  keeps  accurate  record  of  the  regulation,  and  in 
the  sub-station  it  serves  a  similar  purpose. 

An  instrument  sometimes  used  of  late  is  a  frequency  meter, 
showing  on  its  dial  the  periodicity  at  any  time  just  as  an 
ammeter  shows  the  current.  Its  principle  is  very  simple. 
Any  voltmeter  having  some  considerable  reactance  will  change 
its  reading  with  change  of  frequency.  If  furnished  with 
a  scale  empirically  graduated  for  different  frequencies  it 
becomes  a  frequency  meter,  and  if  installed  where  the  voltage 
is  fairly  constant  and  designed  so  as  to  be  hypersensitive  to 
changes  of  frequency  it  may  serve  a  useful  purpose  in  telling 
whether  the  machines  are  at  the  exact  speed  intended.  In  fact 
it  could  in  any  given  situation  be  graduated  for  speed  as  well 
as  for  frequency. 

Occasionally  recording  wattmeters,  similar  to  the  recording 
voltmeters  already  described,  are  used;  but  it  is  difficult  to  get 
accurate  readings  over  a  wide  enough  range  to  be  of  much  use, 
and  the  more  usual  instrument  is  the  integrating  wattmeter, 
sometimes  referred  to  as  recording,  which  registers  the  out- 
put in  watt  hours  continuously.  Instruments  of  this  class  are 
used  both  to  register  the  energy  supplied  to  customers  and  to 
take  account  of  the  energy  generated.  Daily  readings  of  the 
switch-board  instruments  give  by  difference  the  daily  output 
in  KW  hours,  and  in  steam  driven  stations  are  most  important 
in  keeping  record  of  the  station  efficiency  and  its  variations. 


502  ELECTRIC    TRANSMISSION  OF  POWER. 

Even  in  hydraulic  stations  they  give  a  useful  check  on  station 
operation  and  on  the  energy  sold. 

Integrating  wattmeters  are  essentially  motors  whose  speed 
is  proportional   to  the  output.     Like    indicating   wattmeters 


FIG.  266. 

they  produce  a  torque  due  to  the  co-action  of  current  and 
potential  coils,  and  the  armatures  revolving  under  this  stress 
are  furnished  with  an  automatic  drag  due  to  a  disc  revolving 
between  magnet  poles  or  to  air  vanes,  so  that  the  speed  shall 
be  proportional  to  the  output  on  the  circuit  in  watts.  Prob- 
ably in  principle  the  simplest  of  these  instruments  is  the  widely 
known  Thomson  recording  wattmeter.  Fig.  266  shows  the 
general  appearance  of  this  meter  with  the  cover  removed,  and 
Fig.  267  gives  its  connections  in  the  ordinary  two-wire  form. 
Essentially  it  consists  of  the  following  parts:  a  pair  of  field 
coils  of  thick  wire,  in  series  with  the  load;  an  armature,  drum 
wound  of  very  fine  wire,  in  series  with  a  large  resistance  and 


THE  MEASUREMENT  OF  ELEC7'RICAL   ENERGY.      583 

placed  across  the  mains;  and  a  copper  disc  on  the  armature 
shaft  revolving  between  the  poles  of  three  drag  magnets. 
The  fields  and  armature  are  entirely  without  iron,  the 
armature  shaft  rests  on  a  sapphire  jewel  bearing,  and  its  upper 
end  carries  a  worm  to  drive  the  recording  gear. 

The  commutator  is  of  silver,  and  current  is  taken  to  it  by 
slender   copper  brushes   resting   tangentially  upon   it.     The 


FIG.  267. 

drag  magnets  are  artificially  aged,  so  that  they  remain  very 
permanent  and  are  adjustable  to  regulate  the  meter,  if  neces- 
sary. The  resistance  of  the  potential  circuit  is  several  thou- 
sand ohms,  and  the  loss  of  energy  in  the  meter  at  full  load 
does  not  often  exceed  5  to  10  watts.  As  the  static  friction  of 
the  armature  is  considerably  greater  than  the  running  friction, 
the  "  shunt  "  in  the  potential  circuit  is  made  part  of  the  field, 
so  as  to  help  the  meter  in  starting.  Doubling  the  current 
evidently  doubles  the  torque  in  such  a  motor  meter,  but  since 
the  work  done  in  eddy  currents  in  the  drag  increases  as  the 
square  of  the  speed,  the  armature  will  run  at  a  speed  directly 
proportional  to  the  current,  which  is  the  speed  desired. 

In  point  of  fact,  such  meters  are  capable  of  giving  very 
great  accuracy — within  two  per  cent,  under  ordinary  good 
commercial  conditions,  and  very  uniform  results  under  differ- 
ent conditions  of  load. 

Such  meters  are  suited  for  use  on  both  continuous  and 
alternating  circuits,  and  are  remarkably  reliable  in  their  indi- 


ELECTRIC    TRANSMISSION   OF  POWER. 

cations  under  all  sorts  of  conditions.  Another  and  very 
beautiful  group  of  meters  is  designed  especially  for  use  on 
alternating  circuits  only,  and  follows  the  principle  of  the  induc- 
tion motor,  just  as  the  Thomson  meter  is  a  commutating 
motor.  The  pioneer  of  this  class  was  the  famous  Shallenberger 
meter,  an  ampere-hour  meter,  which  has  been  very  widely  used 


FIG.  268. 


and  is  extremely  useful  where  amperes  rather  than  watts  are 
to  be  measured. 

A  fair  type  of  the  induction  wattmeter  is  shown  in  Fig.  268, 
the  Scheefer  meter,  one  of  the  earliest  of  the  class,  although 
here  shown  in  a  recent  form.  It  consists  of  a  finely-laminated 
field  magnet  energized  by  a  current  coil  and  a  potential  coil, 
an  aluminium  disc  armature,  and  the  magnetic  drag  which  has 
come  to  be  generally  used  in  meters.  A  priori  one  would 
suppose  that  so  simple  a  structure  could  hardly  be  made  to 
give  an  armature  speed  proportional  to  the  energy  in  the 
circuit;  and  in  fact  it  takes  great  finesse  to  design  it  so  as  to 
accomplish  this  result,  but  it  can  be  successfully  done,  and 


THE  MEASUREMENT  OF  ELECTRICAL  ENERGY.      585 

meters  of  this  class  turned  out  by  various  manufacturers  are 
capable  of  doing  very  accurate  work. 

As  a  class  they  develop  very  small  torque,  but  in  part  make 
up  for  this  failing  by  the  very  small  weight  of  armature  and 
shaft.  The  speed  is  seldom  accurately  proportional  to  the 
energy  over  a  wide  range  of  load,  but  day  in  and  day  out  the 
small  errors  generally  tend  to  balance  each  other,  so  that 
the  total  reading  at  the  end  of  a  month  varies  but  little  from 
the  facts.  The  induction  meters  are  liable  to  material  errors 
in  case  of  change  of  voltage,  power  factor,  or  frequency,  but 
within  the  range  of  these  factors  in  ordinary  service  they  do 
sufficiently  accurate  work  for  most  purposes. 

All  types  of  meters  are  made  suitable  for  switch-board  work 
in  measuring  large  outputs,  arid  in  alternating  stations  can  be 
fitted  for  use  on  primary  circuits,  although  this  is  seldom 
necessary,  and  should  not  be  attempted  at  any  but  moderate 
voltages  without  the  use  of  transforming  apparatus  for  the 
meter.  Most  switch-board  meters  for  such  work  as  power 
transmission  are  of  special  designs,  modified  for  the  particular 
work  in  hand. 

Monophase  alternating  circuits  and  continuous  current 
circuits  are  measured  in  the  most  direct  way  possible,  the 
ammeters  being  put  in  the  mains,  the  voltmeters  across  them, 
through  a  potential  transformer  if  need  be,  as  it  is  somewhat 
troublesome  to  wind  voltmeters  for  use  directly  upon  circuits 
of  3,000  volts  or  more.  Generally  this  potential  transformer 
is  used  on  all  circuits  above  2,000  to  2,500  volts.  It  is  likewise 
common  to  use  current  transformers,  the  primary  coil  of  which 
is  in  one  of  the  mains,  for  connecting  the  ammeters  in  circuits 
of  higher  voltage  than  just  noted. 

Such  current  transformers  when  calibrated  with  their 
respective  ammeters  can  be  depended  upon  to  give  correct 
indications  at  the  frequency  at  which  they  are  designed  to  be 
used,  but  they  are  not  interchangeable,  although  very  con- 
venient for  use  at  voltages  for  which  it  is  difficult  to  insulate 
the  instruments. 

Wattmeters  are  connected  to  such  circuits  in  a  similar 
straightforward  way,  shown  for  continuous  or  secondary  alter- 
nating current  in  Fig.  267  and  for  primary  alternating  circuits 
in  Fig.  269.  In  these  and  other  cuts  of  wattmeter  connections 


$36 


ELECTRIC   TRANSMISSION  OF  POWER. 


the  circuits  of  the  Thomson  meter  are  shown,  but  they  must 
be  regarded  as  merely  typical,  since  in  using  other  meters  the 
arrangement  of  circuits  follows  the  same  principle,  the  field  or 
current  coil  being  put  in  the  mains  and  the  armature  or  poten- 
tial circuit  across  them.  The  former  is  wound  with  coarse 


GENERATOR 


FIG.  269. 

wire  or  copper  strips,  the  latter  with  very  fine  wire,  so  that 
they  can  very  easily  be  told  apart  even  at  a  casual  inspection. 

Ordinary  two-phase  circuits  are  measured  in  a  precisely 
similar  fashion,  each  pair  of  phase  wires  being  treated  as  a 
separate  circuit  and  supplied  with  its  own  instruments.  The 
metering  likewise,  whether  of  primary  or  secondary  circuits,  is 
accomplished  by  the  use  of  two  wattmeters,  each  connected 
to  its  own  pair  of  mains.  In  case  of  motors  in  which  the  two 
phases  may  be  regarded  as  substantially  balanced  and  equal,  it 
is  only  necessary  to  put  the  instruments  in  one  of  the  phases 
and  to  multiply  the  readings  of  energy  by  2  to  get  the  total 
input.  If  the  two-phase  circuits  are  unbalanced  two  sets  of 
instruments  are  absolutely  necessary  for  a  simultaneous  read- 
ing on  both  phases. 

With  three-phase  circuits  the  case  is  rather  more  compli- 
cated. The  simplest  to  manage  is  a  star-connected  three- 
phase  balanced  circuit,  as  found  in  some  motors.  Here  the 
ammeter  or  current  coil  of  the  wattmeter  goes  directly  into 
one  lead,  and  the  voltmeter  or  potential  coil  of  the  wattmeter 
is  connected  between  that  lead  and  the  neutral  point  of  the  star. 


THE   MEASUREMENT  OF  ELECTRICAL   ENERGY.      587 

The  instruments  then  give  correctly  one-third  of  the  energy 
in  case  of  the  wattmeter  and  one-third  the  current  in  case  of 
the  ammeter.  Therefore  the  wattmeter  reading  multiplied  by 
3  gives  the  energy  on  the  circuit.  On  some  of  the  early  three- 
phase  motors  of  which  the  primaries  were  star-wound,  an  extra 
lead  was  brought  from  the  neutral  point  to  the  connection 
board  to  facilitate  measurements. 

If  the  circuit  is  balanced  it  is  not  necessary  that  a  star  con- 
nection at  the  generator  or  transformers  should  either  be  easily 
accessible  or  exist  in  order  to  use  the  method  of  measure- 
ment just  described.  For  if  the  circuit  is  balanced  the 
ammeter  or  current  coil  of  the  wattmeter  may  be  put  in  a 
lead,  and  the  voltmeter  or  potential  coil  of  the  wattmeter  be 
connected  between  the  same  lead  and  the  neutral  point  formed 
by  three  equal  high  resistances  connected  to  the  three  leads 
respectively,  and  with  their  three  free  ends  brought  to  a  com- 
mon junction.  Such  an  artificial  neutral  is  very  commonly 
used  in  connecting  wattmeters  on  the  secondary  circuits  for 
motors,  and  may  be  applied  to  primary  circuits  as  well.  The 
writer  has  sometimes  constructed  such  a  neutral  by  connect- 
ing three  strings  of  incandescent  lamps  to  the  three  leads  and  to 
a  common  junction.  Then  connecting  the  potential  coil  of  a 
wattmeter  around  one  lamp  and  its  current  coil  in  the  lead  to 
which  the  string  containing  this  lamp  ran,  it  became  possible 
to  make  a  closely  approximate  measurement  of  the  primary 
energy  with  only  an  ordinary  no  volt  meter  and  such  appli- 
ances as  can  be  picked  up  around  any  station.  This  device 
of  an  artificial  neutral  as  applied  to  secondary  circuits  is  well 
shown  in  Fig.  270. 

The  measurement  of  energy  on  an  unbalanced  three-phase 
circuit  is  a  very  different  proposition.  Of  course  three  watt- 
meters with  their  three  potential  coils  respectively  in  the  three 
branches  of  a  star-connected  resistance,  such  as  just  been 
shown,  would  do  the  work,  but  at  a  very  undesirable  cost  and 
complication. 

If,  however,  two  wattmeters  are  used  with  their  current  coils 
in  two  phase  wires  respectively,  and  their  potential  coils 
respectively  between  their  own  phase  wires  and  the  remaining 
wire  of  the  three,  the  sum  of  the  readings  of  these  two  meters 
records  correctly  the  total  energy  of  the  circuit.  Such  an 


588 


ELECTRIC   TRANSMISSION  OF  POWER. 


arrangement  of  meters  is  shown  in  Fig.  271,  as  commonly 
applied  to  three-phase  secondary  circuits.  A  precisely  simi- 
lar arrangement  with  the  addition  of  potential  transformers 
is  used  for  primary  circuits. 

In  a  similar  connection  two  indicating  wattmeters  will  give 
the   energy   of   the   circuit  at   any   moment.     An   indicating 


ri 

! 

i 

3C 

j 

i 
i 

f 

i 

3 

1 

i 
1 
i 
i 
j. 

\\ 

i 

II 

1 

* 

FIG.  270. 

wattmeter  with  its  current  coil  in  one  phase  wire  of  a  three- 
phase  system  will  give  three  diverse  readings  according  as  its 
potential  coil  is  connected  between  its  own  wire  and  each 
of  the  other  phase  wires,  or  finally  across  the  two  other 
phase  wires.  The  latter  reading  is  dependent  on  the  angle  of 
lag,  being  zero  for  unity  power  factor,  and  a  watt  meter  so  con- 
nected can  be  used  as  a  phase  meter,  while  the  two  other  read- 
ings will  be  respectively  increased  and  diminished  to  an  amount 
dependent  on  the  lag. 

Many  attempts  have  been  made  to  combine  the  two  watt- 
meters necessary  to  measure  correctly  the  energy  on  an 
unbalanced  three-phase  circuit  into  a  single  instrument,  and 
with  a  fair  degree  of  success,  but  they  have  generally  been 
rather  cumbersome  and  are  not  commercially  used  to  any 
considerable  extent  in  this  country.  If  the  three-phase  cir- 
cuit to  be  measured  be  a  balanced  one,  such  a  composite  watt- 


THE  MEASUREMENT  OF  ELECTRICAL   ENERGY.      589 

meter  need  merely  have  a  current  coil  connected  in  either 
lead  and  a. pair  of  potential  coils  connected  from  this  to  the 
adjacent  leads  respectively.  In  testing  motors  one  can  readily 
get  the  same  result  if  the  load  be  uniform,  by  using  an  indi- 
cating wattmeter  with  one  lead  connected  through  its  current 
coil  and  then  switching  the  potential  connection  successively 
to  the  adjacent  leads,  and  adding  the  two  readings.  Instru- 


FIG.  271. 

ments  for  unbalanced  circuits  should  have  two  current  and 
two  potential  coils,  as  already  indicated. 

It  should  be  noted  that  in  balanced  mesh-connected  circuits 
one  can  measure  the  energy  correctly  by  putting  the  current 
coil  of  the  wattmeter  into  one  side  of  the  mesh  inside  the  joint 
connection  to  the  lead,  and  the  potential  coil  across  the  same 
side  of  the  mesh.  This  gives  a  reading  of  one-third  the  total 
energy.  Ammeter  and  voltmeter  similarly  connected  give 
readings  showing  one-third  the  apparent  watts. 

In  ordinary  three-phase  distributing  systems  the  actual 
metering  is  much  simpler  than  would  appear  at  first  sight. 
Motors  are  provided  with  a  single  meter,  usually  connected  as 
shown  in  Fig.  270.  Much  of  the  lighting  is  from  a  pair  of 
phase  wires  or  from  one  phase  wire  and  the  neutral,  in  which 
case  the  secondary  service  is  a  simple  two-wire  distribution 
measured  like  any  other  monophase  system.  In  cases  where 
all  three  wires  are  taken  into  the  same  service  the  energy 
can  be  measured  by  two  meters,  as  shown  in  Fig.  271. 


59°  ELECTRIC   TRANSMISSION"  OF  POWER. 

The  induction  type  of  meter  is  sometimes  liable  to  consider- 
able errors  on  motor  circuits  where  the  power  factor  is  subject 
to  large  variations,  and  should  therefore  be  used  with  caution. 
Before  purchasing  meters  it  is  advisable  to  ascertain  by  actual 
tests  how  they  will  perform  on  circuits  of  varying  power  factor. 

Meters  should  be  installed  where  they  will  be  free  from 
vibration,  extreme  heat  and  dampness,  chemical  fumes,  and 
dust.  To  a  less  extent  the  same  rules  apply  to  other  instru- 
ments, but  meters  with  their  constantly  moving  parts  and 
very  light  torque  should  be  looked  after  with  particular  care. 
They  should  be  inspected  every  few  months,  and  at  less 
frequent  intervals  should  be  tested  in  situ,  which  can  very 
readily  be  done  by  the  aid  of  an  indicating  wattmeter  con- 
nected to  the  same  load.  The  following  formula  serves  for 
this  test: 

3600  X  Constant  of  meter  (if  any)  _ 
Watts  in  use 

seconds  per  revolution  of  armature. 

Nearly  all  meters  use  the  magnetic  drag,  and  a  light  mark 
near  the  periphery  of  the  meter  disc  timed  for  a  few  revolu- 
tions with  a  stop  watch  gives  the  right  hand  side  of  the  equa- 
tion, while  the  watts  input  is  checked  by  the  indicating 
wattmeter.  The  constant  of  the  meter  by  which  its  reading 
must  be  multiplied  to  give  the  true  energy  recorded  is  nearly 
always  plainly  marked  as  an  integral  number  upon  the  meter. 
If  a  meter  shows  material  error  it  can  be  brought  to  the  cor- 
rect rate  by  slightly  shifting  the  position  of  a  drag  magnet  or 
adjusting  the  dragging  device,  whatever  it  is.  This  adjust- 
ment up  to  a  reasonable  amount  is  provided  for  in  meters  of 
all  types,  and  if  the  error  is  more  than  can  be  thus  compen- 
sated the  meter  should  be  thoroughly  overhauled,  particularly 
as  to  the  armature  bearings. 

With  proper  care  meters  in  commercial  service  can  be  kept 
correct  within  two  or  three  per  cent,  year  in  and  year  out. 
They  are  more  apt  to  run  slow  than  fast,  so  that  the  consumer 
seldom  has  just  ground  for  complaint.  For  the  best  work 
meters  should  be  installed  with  the  idea  of  keeping  them 
generally  working  near  their  rated  loads.  The  greatest  inac- 
curacies are  at  light  loads,  and  part  of  the  inspector's  duty 


T11K   MEASUREMENT  OF  ELECTRICAL   ENERGY.      591 

should  be  to  make  certain  that  the  consumer's  meter  will  start 
promptly  on,  say,  a  single  8  c.  p.  incandescent  lamp.  Other- 
wise the  consumer  can,  and  usually  finds  out  that  he  can,  get 
a  certain  amount  of  light  without  paying  for  it.  Electric 
meters  nearly  always  are  read  on  their  dials  in  exactly  the 
manner  that  gas  meters  are  read.  With  unskilled  or  careless 
men  reading  the  meters  there  is  some  chance  for  mistake. 
To  avert  this  some  companies  furnish  their  meter  readers 
with  record  books  having  facsimiles  of  the  meter  dials  plainly 
printed  on  the  pages.  The  reader  then  merely  marks  on  these 


Customer Meter  No. 

Meter  Capacity Rate Constant. 


Jan. 


FIG.  272. 

with  a  sharp-pointed  pencil  the  position  of  the  hand  on  each 
dial  of  the  consumer's  meter,  and  the  record  thus  made  is 
translated  deliberately  at  the  office.  Part  of  a  page  from 
such  a  record  book  in  shown  in  Fig.  272. 

A  direct-reading  meter,  arranged  somewhat  after  the  man- 
ner of  a  cyclometer,  showing  the  total  reading  in  plain  fig- 
ures, is  a  highly  desirable  instrument,  but  although  several 
such  meters  have  been  brought  out  they  have  not  as  yet  come 
into  a  secure  place  in  the  art.  The  difficulty  is  mainly  a 
mechanical  one.  The  meter  can  easily  work  one  number  disc, 
but,  as  it  runs  on,  an  evil  time  comes  when  it  has  simultane- 
ously to  move  two,  three,  four,  or  five  discs,  and  atone  of  these 
points  it  is  likely  to  balk.  Such  a  meter  would  be  particularly 
hard  to  adapt  to  the  induction  type  now  widely  used,  and, 
desirable  as  it  would  be,  the  time  of  its  coming  is  not  yet. 


592  ELECTRIC   TRANSMISSION   OF  POWER. 

For  special  purposes  a  considerable  variety  of  meters  are 
used,  all,  however,  being  made  and  applied  on  substantially  the 
lines  already  described.  In  some  cities  prepayment  meters 
with  an  attachment  for  switching  on  the  current  worked  like 
a  slot  machine  are  finding  a  foothold,  particularly  in  the 
poorer  quarters.  Elsewhere  two-rate  meters  with  a  clock- 
work attachment  to  cut  down  the  rate  of  running  between 
certain  hours  of  relatively  light  station  load,  and  some  other 
automatic  discount  meters,  have  been  employed.  But  all  these 
are  peculiar  in  their  special  attachments  rather  than  in  any 
fundamentals. 

Whatever  meters  and  instruments  are  used,  it  is  of  primary 
importance  that  they  be  kept  always  in  the  best  working  order. 


CHAPTER   XVII. 

THE    PRESENT    STATE   OF    HIGH-VOLTAGE    TRANSMISSION. 

SINCE  the  appearance  of  the  first  edition  of  this  work  in  1897 
there  has  been  very  rapid  progress  in  the  art  of  dealing  with 
very  high  voltages.  Much  that  was  then  dubious  is  now  cer- 
tain, and  at  the  present  time  engineers  use  ten  or  fifteen  thou- 
sand volts  as  freely  as  they  used  half  that  pressure  three 
years  ago. 

Moreover,  the  experiments  then  just  initiated  with  far  higher 
pressures  have  led  to  a  successful  conclusion,  and  when  the 
distance  of  transmission  demands  extreme  measures  there  is 
now  little  hesitancy  in  employing  voltages  of  20,000  to  40,000  if 
the  climatic  conditions  are  at  all  favorable.  Up  to  two  years 
ago  little  experience  regarding  the  behavior  of  insulators  at 
very  high  pressures  had  been  accumulated,  and  this  was  really 
the  critical  factor  in  dealing  with  the  subject.  It  has  turned 
out  that  with  well-designed  glass  or  porcelain  insulators  of  the 
quality  now  obtainable  there  is  little  danger  of  puncture  or 
serious  leakage  up,  certainly,  to  20,000  volts.  With  the  best 
commercial  insulators,  carefully  tested  previous  to  installation, 
even  this  very  high  voltage  may  be  doubled  without  likeli- 
hood of  serious  trouble,  provided  climatic  conditions  are  good. 
At  such  pressures  some  curious  phenomena  develop  and  radi- 
cally new  conditions  are  encountered,  but  everything  points 
to  their  being  successfully  met.  There  are  at  present  run- 
ning in  the  United  States,  Canada,  and  Mexico  nearly  seventy 
transmission  plants  working  regularly  at  voltages  of  10,000  or 
more;  and  of  these,  a  score  are  operating  at  or  above  20,000 
volts.  These  last  bear  the  same  relation  to  future  progress 
in  high  voltage  work  that  the  little  group  of  io,ooo-volt  plants 
bore  five  years  since. 

Much  of  our  knowledge  of  the  subject  is  due  to  the  exhaustive 
tests  made  by  Mr.  Ralph  D.  MershonatTelluride,  Col.,  briefly 
referred  to  in  Chap.  XII.  These  are  fully  described  in  a  paper  by 


594  ELECTRIC   TRANSMISSION   OF  POWER. 

Mr.  C.  F.  Scott  before  the  American  Institute  of  Electrical  En- 
gineers.  Briefly  the  essential  points  established  are  as  follows: 

With  first-class  glass  or  porcelain  insulators  there  is  little 
to  fear  in  the  way  of  leakage  in  good  weather  up  to  say  50,000 
volts,  unless  the  insulators  break  from  mechanical  causes. 
Rain  and  snow  seldom  cause  trouble,  although  of  course  they 
may  do  so.  Dry  air,  snow,  and  clean  water  are  tolerable  insu- 
lators, although  dirt  of  any  kind  on  the  insulators  is  to  be 
feared.  Cross-arms  and  pins  should  be  filled  to  prevent  infil- 
tration of  moisture.  There  is  good  reason  to  believe  that  oil 
insulators  are  quite  needless,  and  they  certainly  are  apt  to 
accumulate  dirt  without  giving  any  compensating  advantage. 

Loss  of  energy  through  leakage  over  the  surface  of  the  insu- 
lator is  very  trifling — perhaps  not  more  than  two  or  three  watts 
per  insulator,  even  at  50,000  volts  between  lines.  In,  bad 
weather  this  is  likely  to  increase;  but  the  leakage  itself  tends 
to  dry  the  surface  of  the  insulators,  and  while  if  high  voltage  is 
suddenly  thrown  on  a  line  leakage  may  cause  immediate  trou- 
ble, gradually  raising  the  pressure  to  the  full  working-point 
tends  to  correct  this  difficulty,  and  there  is  no  trouble  ex- 
perienced. Rain  is  the  only  thing  likely  to  cause  difficulties, 
unless  we  except  a  heavy  snowstorm,  and  neither  is  much  to  be 
feared  at  40,000  to  50,000  volts,  although  above  this  point 
troubles  increase  rapidly.  A.  heavy  sea-fog,  with  salt-laden 
atmosphere,  is  serious,  however,  as  has  been  shown  on  lines 
of  far  lower  voltage  than  the  above,  and  sometimes  has  caused 
the  burning  of  pins,  cross-arms,  and  even  poles.  Sappy  cross- 
arms  and  poles  have  been  a  not  uncommon  source  of  damage. 

The  most  interesting  fact  brought  out  in  the  Telluride 
experiments  was  the  leakage  at  very  high  voltage  between  wire 
and  wire  through  the  air — a  true  brush  discharge  akin  to 
that  seen  about  the  wires  proceeding  from  a  big  induction  coil. 
At  about  20,000  volts  the  line  wires  begin  to  show  at  night 
traces  of  luminosity,  which  rapidly  increases  with  the  pressure, 
until  at  50,000  volts  and  upward  the  wires  are  plainly  visible 
for  a  long  distance,  and  the  hissing  sound  characteristic  of 
fierce  brush  discharges  is  audible  at  the  distance  of  a  hundred 
feet  or  more.  This  discharge  involves  a  real  and  quite  consid- 
erable loss  of  energy,  increasing  with  appalling  rapidity  at  still 
higher  voltages.  This  loss  increases  with  rise  of  voltage,  with 


HIGH.  VOL  TA  GE    TRANSMISSION. 


595 


diminution  of  the  distance  between  wires,  and  with  diminution 
of  the  diameter  of  wire.  Anything  on  or  about  the  wire  which 
increases  the  electric  density  at  any  point  shows  as  a  noticeable 
brush  discharge.  Fig.  273  shows  the  loss  on  the  Telluride  cir- 


4500 


4000 


3500 


9000 


fcOO 


2000 


MOO 


WOO 


Loss  on  Circuit  with  Wires  at 

Different  Distances. 

frequency  60;  Slotted  Armature 

Wires  15,  22,  35  and  52  inches 

apart. 


12        16        20 


24        28        32        36 
Thousands  of  Volts 
FIG.  273. 


40 


48 


cuit,  2%  miles  in  total  length,  with  line  wires  at  different  dis- 
tances apart  on  the  poles.  The  curves  begin  to  turn  upward 
at  between  40,000  and  50,000  volts,  and,  once  the  elbow  of  the 
curve  is  passed,  rise  very  fast. 


59* 


ELECTRIC   TRANSMISSION  OF  POWER. 


It  is  clear  that  this  phenomenon  sets  a  real  physical  limit  on 
high-voltage  transmission,  of  a  sort  not  hitherto  realized.  A 
loss  of  say  2  KW  per  mile  of  wire  is  a  pretty  serious  matter 
on  a  long  line,  even  when  the  total  energy  transmitted  is 
very  considerable,  and  obviously  in  any  practical  case  of 
transmission  the  voltage  must  be  kept  well  below  the  elbow  of 


.45  4500 


.4  4000 


.35 


8000 


2500 


.2  2000 


.15  1500 


4  1000 


SMOOTH  and  TOOTHED  ARMATURES 

Current  and  Loss  for  Smooth  Armature. 

Frequency  30;  Thomson  Wattmeter 
Single  Cir. Two  Circuitsin  Multiple 

Loss  for  Toothed  Armature. 

Frequency  60. 

1  —  By  Thomson  Wattmeter 
2 By  W'tm'rin  primary  of  Raising  Transf! 


0       4        8       12162084233286404448525660646872 
Thousands  of  Volts 
FIG.  274. 

the  leakage  curve.  Hence  the  wires  should  be  kept  well 
apart.  As  might  have  been  expected,  the  shape  of  the 
E.  M.  F.  wave  given  to  the  line  is  a  very  important  matter 
under  these  circumstances.  Fig.  274  illustrates  this  fact 
in  a  sufficiently  startling  manner.  By  changing  from  a 
non-sinusoidal  wave  of  the  peaked  type  to  a  wave  nearly 
sinusoidal  the  elbow  of  the  curve  was  pushed  up  from  40,000 
volts  to  60,000  volts  under  conditions  otherwise  the  same.  P'or 


HIGH-VOLTAGE    TRANSMISSION1.  597 

such  work  there  is  every  possible  reason  for  using  the  sine 
wave  on  account  of  this  leakage  effect  as  well  as  for  other 
practical  reasons  already  set  forth.  But  even  with  favoring 
conditions  leakage  must  be  reckoned  with,  and  present  data 
indicate  that  in  nearing  50,000  volts  one  is  on  dangerous  ground 
quite  aside  from  any  questions  of  insulators  or  apparatus.  We 
cannot  expect  to  improve  the  dielectric  strength  of  the  air, 
and  although  the  conditions  may  be  improved  by  using  good- 
sized  wires  when  conditions  permit,  or  perhaps  by  heavily  in- 
sulating the  wires,  thus  both  increasing  their  diameter  and 
somewhat  improving  the  general  insulation,  it  is  quite  prob- 
able that  a  limitation  of  voltage  from  this  cause  is  an  un- 
pleasant reality. 

With  proper  precautions,  however,  there  seems  to  be  no 
very  grave  difficulty  in  working  at  40,000  to  50,000  volts. 
Besides  the  Telluride  experiment,  in  which  the  short  line  men- 
tioned was  worked  steadily  for  a  month  at  50,000  volts  without 
serious  interruptions,  there  has  now  been  in  operation  for  more 
than  two  years  a  commercial  transmission  over  55  miles  at  a 
working  pressure  of  40,000  volts.  This  is  the  plant  of  the 
Telluride  Power  Transmission  Co.  at  Provo,  Utah.  The 
power  is  utilized  for  driving  the  Mercur  mills  and  other  work 
in  connection  with  a  large  mining  property.  The  generating 
plant  consists  of  a  pair  of  750  KW  three-phase  generators 
directly  connected  to  turbines.  They  run  at  300  r.  p.  m., 
giving  6o~,  800  volts.  The  raising  transformers  of  250  KW 
each  are  set  up  in  the  star  connection  with  the  neutral  points 
of  both  primary  and  secondary  grounded.  The  line  involves  a 
rather  tough  bit  of  mountain  construction,  reaching  an  ex- 
treme altitude  of  over  10,000  feet. 

Both  glass  and  porcelain  insulators  are  in  practical 
use  on  very  high  voltage  lines.  The  former  of  course 
show  defect  by  inspection,  while  the  latter  must  be  tested 
previous  to  installation  to  avert  serious  trouble.  In  fact, 
few  actually  do  break  down  under  test,  but  if  these  few 
are  on  the  line  the  results  may  be  unpleasant.  On  the 
Provo  system  glass  insulators  are  used,  the  form  adopted 
being  that  shown  in  Fig.  275.  It  is  of  triple  petticoat  design, 
7*  in  diameter  over  all  and  5^"  high.  The  lower  edge  of  the 
outer  petticoat  is  toothed  to  prevent  the  accumulation  of 


593 


ELECTRIC    TRANSMISSION  OF  POWER. 


water.  The  water  gathers  on  the  points  and  quickly  drops 
off.  These  insulators  have  done  well  upon  the  whole, 
although  there  is  always  enough  leakage  to  prevent  the 
successful  working  of  a  telephone  line  on  the  same  poles. 
This  may  not  be  wholly  chargeable  to  the  material  of  the 
insulators,  as  the  climatic  conditions  are  extremely  trying. 


FIG.  275. 

The  western  end  of  the  line  lies  near  the  old  basin,  of  the 
Great  Salt  Lake,  where  the  dust  is  of  strongly  saline  character, 
and  occasionally  so-called  "  salt-storms"  occur,  which  would 
severely  try  the  insulation  of  any  imaginable  line.  The  saline 
matter  gets  distributed  over  not  only  the  insulators,  but  the 
cross-arms,  pins,  and  poles,  and  is  sufficient  not  only  to  ensure 
leakage,  but  to  cause  actual  arcs  to  break  out  between  wire 
and  wire,  producing  a  momentary  short  circuit.  Such  arcs 
quickly  dissipate  themselves,  but  while  they  last  the  phe- 
nomenon is  little  short  of  terrific.  Sometimes,  though  rarely, 
in  conditions  of  unusual  severity  these  tremendous  discharges 
come  for  a  short  time  in  rapid  succession. 

Under  such  circumstances  the  photograph  reproduced  in 
Plate  XVIII  was  taken,  and  when  one  realizes  that  all  three  wires. 


PLATE  XVIII. 


HIGH-VOLTAGE  TRANSMISSION.  599 

were  involved,  and  that  the  pole  is  nearly  forty  feet  high,  some 
idea  of  the  imposing  character  of  the  result  may  be  obtained. 
The  distance  from  wire  to  wire  is  a  trifle  over  six  feet,  so 
that  the  flaming  arc  is  upward  of  ten  feet  in  extent.  Fortu- 
nately such  salt  storms  are  infrequent,  and  comparatively 
little  damage  is  done  to  the  line,  but  considering  the  peculiar 
conditions  it  is  not  remarkable  that  leakage  is  perceptible. 
Saline  matter,  whether  as  dust  or  from  a  heavy  sea  fog,  is  the 
worst  enemy  of  insulation. 

On  the  33,ooo-volt  Redlands-Los  Angeles  line  porcelain  in- 
sulators on  iron  pins  are  used,  and  leakage  has  been  very  little 
felt,  there  being  not  enough  to  disturb  the  telephone  service 
to  any  material  degree.  The  insulators  are  of  about  the  same 
size  as  the  Provo  type,  and  were  very  thoroughly  tested  prior 
to  installation.  They  have  proved  remarkably  good,  and 
actual  puncture  is  practically  unknown  both  here  and  at 
Provo.  How  much  of  the  lessened  trouble  on  the  Redlands- 
Los  Angeles  system  is  due  to  the  lower  voltage  than  at  Provo, 
how  much  to  the  use  of  porcelain,  and  how  much  to  the  better 
climatic  conditions  it  is  quite  impossible  to  tell. 

The  use  of  iron  pins  the  writer  considers  bad  practice  on  very 
high  voltage  lines,  in  spite  of  their  strength  and  convenience. 
It  gives  altogether  too  good  a  path  to  the  cross-arm  in  case  of 
leakage  or  puncture,  and  has  no  large  compensating  advan- 
tages. On  lines  of  10,000  to  20,000  volts  under  ordinary  con- 
ditions the  insulator  just  described  in  connection  with  the 
Provo  plant  is  needlessly  large,  and  a  smaller  type  of  the  same 
design  shown  in  Fig.  276  has  come  into  considerable  use.  It 
is  5^"  in  diameter  and  4^"  high. 

In  one  particular  both  these  insulators  are  open  to  criticism. 
The  wire  is  of  necessity  tied  to  the  side  of  the  insulator  top 
instead  of  resting  in  a  top  groove,  as  in  the  insulator  shown 
in  Fig.  277.  This  lateral  tie  lessens  the  leakage  distance  to 
the  pin  very  materially,  and  in  the  worst  possible  place — that 
is,  where  the  insulator  is  smallest  in  diameter,  and  hence  offers 
the  least  surface.  For  high  voltage  work  the  top  groove  is 
decidedly  preferable. 

On  the  two  great  California  high-voltage  lines,  those  of  the 
Standard  Electric  Co.  and  the  Bay  Counties  Power  Co.,  both 
designed  for  an  ultimate  working  pressure  of  60,000  volts,  the 


600  ELECTRIC   TRANSMISSION  OF  POWER. 

immense  striking  distance  of  this  extreme  pressure  forced  the 
design  of  special  insulators.  The  form  settled  upon  is  shown 
in  Fig.  277.  This  is  of  composite  construction.  The  upper 
part  is  a  spreading  porcelain  umbrella  with  a  pair  of  diametri- 
cally opposite  lips  and  a  strong  upper  head,  with  top  groove 
and  groove  for  the  tie  wire.  This  porcelain  umbrella  is  12" 


FIG.  276. 

in  diameter,  and  is  cemented  with  sulphur  upon  the  glass  base, 
which  has  a  single  petticoat  of  small  diameter,  12"  deep.  The 
whole  is  fitted  for  an  extra  thick  and  long  pin. 

These  insulators  are  tested  with  120,000  volts  between  the 
tie  wire  and  the  centre  of  the  pin.  This  pressure,  with  a  nor- 
mal striking  distance  of  about  12"  in  air  between  points,  fails 
to  puncture  the  insulators,  but  may  discharge  over  the  surface. 
The  pins  are  oil  treated,  as  is  usual  with  pins  intended  for 
high-tension  work. 

As  the  lines  to  which  these  insulators  belong  are  not  yet 
regularly  worked  at  the  full  voltage,  but  at  40,000  volts,  data 
as  to  the  real  practical  properties  of  the  insulators  are  wanting. 
They  seem,  however,  to  be  admirably  adapted  to  their  work, 
although  the  very  large  surface  of  the  umbrella  would  appear 
to  give  a  resistance  to  leakage  much  smaller  than  would 
result  from  a  bell-shaped  design.  The  composite  structure  is 
much  easier  to  manufacture  than  if  the  whole  mass  were  solid, 


HIGH-  VOL  TA  GE    TRA  NSMISSION. 


601 


and  it  is  comparatively  easy  to  make  a  high  grade  of  porcelain 
in  the  shape  chosen. 

All    the    information    now  accumulated   with    reference  to 
insulating   very    high   voltages    indicates   that    the    task    is 


FIG.  277. 


easier  than  has  commonly  been  supposed.  The  main  thing 
seems  to  be  to  allow  ample  space  between  the  wire  and  its 
supports  and  between  wire  and  wire.  A  point  must  ultimately 
be  reached  at  which  the  cost  of  proper  insulation  will  balance 
the  saving  in  line  wire  made  by  adopting  that  particular 


602  ELECTRIC   TRANSMISSION  OF  POWER. 

voltage.     This  point  is  apparently  above  40,000  volts,  but  how 
much  above  it  is  impossible  at  present  to  say. 

The  Provo  plant  has  done  good  steady  work  ever  since  it 
was  started.  It  has  not  been  exempt  from  troubles,  of  course, 
but  there  have  been  no  serious  breakdowns. 

In  ordinarily  dry  weather  the  line  has  uniformly  worked 
well — as  well  as  lines  at  a  much  lower  voltage.  In  rainy 
weather  there  has  been  some  trouble  from  leakage — sometimes 
enough  to  blow  the  fuses.  In  almost  every  instance  of  line 
troubles  the  cause  has  been  found  to  be  a  broken  insulator, 
cracked  from  strain  or  smashed  by  a  bullet.  The  first  time 
the  author  crossed  the  continent  on  power-transmission  busi- 
ness he  received  a  vivid  idea  of  the  bullet  as  a  factor  in  the 
situation,  for  nearly  every  switch  target  between  Kansas  City 
and  Los  Angeles  had  from  one  to  a  dozen  bullet  holes  in  it. 

The  Provo  plant  has  been  in  operation  since  February,  1898, 
and  in  spite  of  the  terrific  voltage  and  occasional  difficulties 
on  the  line,  it  has  done,  and  is  doing,  exceedingly  good  service. 
Such  pressure,  however,  must  be  regarded  as  still  somewhat 
experimental,  and  it  should  be  borne  in  mind  that  the  kind 
of  work  on  which  it  is  employed  is  such  that  trivial  disturb- 
ances are  not  noticed  much,  and  brief  interruptions  at  infre- 
quent intervals  are  not  serious — simply  annoying.  What  a 
similar  plant  would  do  in  the  way  of  furnishing  a  general  ser- 
vice of  light  and  power  is  still  somewhat  problematical. 

It  is  needless  to  say  that  at  extremely  high  voltages, not  only 
the  insulators,  but  the  line  itself,  requires  some  special  con- 
struction. Not  only  must  the  line  wires  be  kept  well  apart, 
but  care  must  be  taken  with  the  cross-arms  and  poles.  The 
line  must  be  kept  most  scrupulously  clear,  for  a  tree  branch 
even  swinging  close  to  the  line  is  liable  to  start  an  arc  to 
ground. 

If  an  insulator  is  broken  a  discharge  is  quite  certain  to 
burn  the  pin  and  ultimately  the  cross-arm  itself,  particularly 
in  damp  weather  or  in  case  of  rather  porous  wood  in  pins  or 
cross-arms.  Filling  the  cross-arms  with  oil,  as  pins  are  filled, 
has  a  salutary  effect  in  checking  this  tendency  to  leakage 
along  the  grain.  The  insulators  should  be  carried  with  their 
edges  at  least  a  full  diameter  of  the  insulator  above  the 
cross-arm. 


HIGH-VOLTAGE    TRANSMISSION. 


603 


A  good  type  of  high-voltage  line  construction  is  shown  in 
the  pole  head  of  Fig.  278.  This  is  from  the  recently  con- 
structed line  of  the  Missouri  River  Power  Co.,  into  Butte, 
Mont.  It  is  a  yo-mile  transmission  at  50,000  volts.  The 
particular  features  of  this  construction  are  the  general  use  of 


FIG.  278. 

bolts  instead  of  lag  screws,  and  the  substitution  of  hard  wood 
for  iron  cross-arm  braces.  This  latter  precaution  consider- 
ably increases  the  insulation  resistance  at  the  pole  head.  The 
thing  which  really  determines  leakage  is  not  so  much  the  dis- 
tance from  wire  to  wire  through  air,  as  the  distance  via  cross- 
arm  and  pole.  For  example,  in  Fig.  278  the  horizontal 
distance  in  air  between  the  two  lower  wires  is  66".  But  the 


604  ELECTRIC   TRANSMISSION  OF  POWER. 

distance  via  the  cross-arm  is  only  something  like  2'  in  air  and 
along  insulator  surface,  plus  about  5'  along  possibly  very  wet 
wood.  This  insulation  would  break  down  at  much  less  volt- 
age than  would  be  required  to  jump  from  wire  to  wire.  If 
the  braces  were  iron  the  discharge  would  have  to  cross  barely 
3'  of  wood  in  its  path  from  wire  to  wire. 

On  long  and  important  transmissions  duplicate  pole  lines 
are  coming  into  general  use.  This  of  course  practically 
doubles  the  total  cost  when  the  voltage  is  high  enough  to 
keep  the  cost  of  the  conductor  low,  but  it  certainly  gives  in 
many  instances  greatly  increased  security  against  interruption, 
especially  where  trees  are  to  be  feared. 

Up  to  10,000  or  15,000  volts  a  well-built  pole  line  with 
duplicate  circuits,  kept  rigorously  on  opposite  sides  of  the 
pole,  answers  very  well  in  clear  country. 

There  is  little  difficulty  in  freely  executing  repairs  on  one 
line  while  the  other  is  in  use,  by  the  exercise  of  ordinary  dis- 
cretion. Sometimes  a  wire  is  wound  a  few  times  around  the 
cross-arm  and  then  grounded,  to  head  off  leakage,  and  no  line- 
man should  be  allowed  anywhere  on  a  pole  carrying  such  cir- 
cuits without  his  wearing  rubber  gloves;  but  on  the  whole 
repairs,  even  under  such  circumstances,  are  not  troublesome. 
In  case  of  need  a  10,000  or  i5,ooo-volt  circuit  can  be  repaired 
while  still  alive,  in  dry  weather,  without  great  difficulty. 

No  plant,  however,  should  be  so  planned  as  to  require  such  a 
proceeding  under  any  ordinary  circumstances. 

As  the  voltage  mounts  to  20,000  or  more,  operations  upon 
a  pole  line  carrying  live  wires  become  difficult  and  some- 
what hazardous,  and  should  never  be  attempted  save  as  a  last 
resort  in  case  of  unusual  accidents. 

Allusion  has  already  been  made  to  a  remarkable  engineer- 
ing feat  on  the  lines  of  the  Bay  Counties  Power  Co.,  in  the 
spanning  of  Carquinez  Straits  by  steel  cables.  It  is  sufficiently 
noteworthy  to  call  for  more  than  a  passing  word, 

The  problem  was  to  cross  a  deep,  swift,  navigable  water- 
way, 3200  feet  wide  at  the  narrowest  point.  Submarine  cables 
were  out  of  the  question,  and  the  United  States  Engineers 
required  200  feet  above  high  water  mark  for  the  lowest  dip 
of  any  suspended  structure. 

On  the  north  shore  on  a  point  160  feet  above  high  water 


PLATE  XIX. 


HIGH-  VO  L  TA  GE    TRA  NSMISSION. 


605 


was  erected  the  skeleton  steel  tower  shown  in  Plate  XIX. 
On  the  south  shore  there  was  higher  land,  and  a  similar  tower 
65  feet  high  sufficed.  The  construction  adopted  was  that  gen- 
erally used  for  steel  tower  work,  and  each  tower  bore  near  its 
top  four  massive  wooden  out-riggers  surmounted  by  the  in- 
sulated saddles  that  carried  the  weight  of  the  cable  spans. 

As  in  suspension  bridge  work  the  cables  rest  upon  rollers 
upon  the  saddles,  and  then  extend  far  shoreward  to  the  anchor- 
ages, where  the  strain  is  taken.  From  anchorage  to  an- 
chorage the  span  is  6,200  feet.  Each  cable  consists  of  nine- 


FIG.  279. 

teen  strands  of  steel,  galvanized,  is  seven-eighths  of  an  inch  in 
diameter  over  all,  and  has  the  electrical  conductivity  of  No.  2 
copper.  The  breaking  strain  of  the  cable  is  98,000  pounds, 
each  span  weighs  7,080  pounds,  and,  as  suspended  with  100 
feet  dip,  has  a  factor  of  safety  of  4. 

Two  difficult  problems  of  insulation  were  presented.  First, 
the  great  weight  of  the  cable  must  be  supported  at  the  saddle 
with  insulation  adequate  for  60,000  volts.  Second,  the  pull 
must  be  taken  at  the  anchorage  with  equally  high  insulation. 


6o6 


ELECTRIC   TRANSMISSION  OF  POWER. 


The  pull  of  the  cable  being  12  tons,  the  task  at  the  anchorage 
was  by  far  the  more  difficult  of  the  two. 

At  the  saddle  the  weight  is  taken  upon  huge  triple  petticoat 
porcelain  insulators,  each  built  up  of  four  great  nested  porce- 
lain cups,  the  inner  being  filled  with  sulphur  securing  a  large 
steel  pin.  Six  such  insulators,  each  17"  in  diameter  over  the 
outer  petticoat,  co-operate  to  sustain  the  pressure  at  each 
saddle.  Fig.  279  shows  a  cross  section  through  insulators, 
supports,  frame,  and  saddle.  The  heads  of  the  insulators  are 
built  into  a  timber  platform  which  serves  at  once  as  a  rain 
shed  and  a  base  for  the  cast  iron  saddle  proper.  This  carries 
in  line  five  steel  grooved  sheaves  over  which  the  cable  passes. 


,  Each  layer  fastened  witfi 
woodeu  dowel-plus 

/\  TS 


FIG.  280. 


Fig.  280  shows  the  structure  in  longitudinal  section,  together 
with  the  suspended  platform  beneath  it,  for  ease  of  access. 

The  strain  insulators  for  the  anchorage  are  of  highly  ingeni- 
ous construction.  Micanite  seemed  to  be  the  only  insulating 
substance  possessing  the  necessary  mechanical  strength,  and 
to  prevent  surface  leakage  across  it  the  surface  exposed  to 
leakage  was  enclosed  in  an  oil  tank.  Fig.  281  shows  the  struc- 


HIGH-  VOL  TA  GE    TRA  NSMISSION. 


607 


ture  of  the  completed  insulator  more  plainly  than  description. 
Two  of  these  insulators  are  put  in  series  and  enclosed  in  a 
shelter  shed  to  keep  off  water,  for  each  cable,  the  pair  being 
secured  to  a  long  tie  rod  anchored  in  a  massive  bed  of 
concrete. 

Great  care  was  taken  in  all  the  details  of  the  structure  to 
secure  all   the  insulation   practicable,  even   the   timber   out- 


FIG.  281. 

riggers  carrying  the  saddle  insulators  being  filled  and  varnished 
and  the  foundation  timbers  proper  being  boiled  in  paraffin. 
The  use  of  four  cables  gives  one  reserve  conductor  in  case  of 
accident.  The  total  length  of  the  span  from  tower  to  tower 
is  4,427  feet,  2^  times  the  span  of  the  Brooklyn  Bridge.  It  is 
one  of  the  most  striking  engineering  feats  in  the  records  of 
electrical  power  transmission,  and  while  not  yet  long  in  service 
it  has  withstood  successfully  very  severe  tests. 

For  nearly  two  years  past  the  large  plant  of  the  Southern 
California  Power  Co.  has  been  in  operation,  delivering  power 
from  a  point  near  Redlands  to  Los  Angeles,  a  distance  of  80 
miles.  This  plant  contains  four  750  KW  generators  directly 
connected  to  impulse  wheels.  The  generators  run  at  300  r.  p. 
m.,  giving  750  volts  at  5o~.  The  transmission  is  by  far  the 
longest  yet  operated  for  any  considerable  time  for  commercial 
purposes,  and  the  results  so  far  have  been  highly  satisfactory. 
The  working  voltage  is  33,000,  and  in  the  favorable  climate  of 
this  part  of  California  there  is  little  reason  to  apprehend  any 
trouble  in  permanent  successful  operation.  This  great  trans- 
mission forms  a  part  of  what  is  perhaps  the  most  remarkable 
development  of  electrical  power  distribution  yet  accomplished. 
Southern  California  is,  in  spite  of  the  abundance  of  fuel  oil 
locally,  a  region  of  dear  fuel,  and  consequently  high  cost  of 


608  ELECTRIC  TRANSMISSION  OF  POWER. 

power.  It  is,  moreover,  favored  with  mountain  streams  along 
the  Sierras,  decidedly  variable  in  amount,  but  capable  of  rather 
cheap  development  for  very  high  heads.  The  climate  is 
favorable  for  power  transmission  projects,  being  very  dry  for 
a  large  part  of  the  year,  comparatively  free  from  severe 
thunderstorms,  and  during  the  season  of  rains  characterized 
by  heavy  showers  rather  than  the  prolonged  drizzles  that  are 
so  trying  to  insulation. 

So  it  has  come  about  that  beginning  with  1892  this  region 
has  been  the  seat  of  highly  interesting  and  successful  power 
enterprises,  so  that  there  is  scarcely  a  town  of  appreciable  size 
in  an  area  of  2,000  square  miles  in  which  electric  power  is  not 
available  at  a  reasonable  price. 

Fig.  282  gives  an  outline  map  of  the  region  involved,  and 
shows  the  remarkable  ramification  of  transmission  lines 
throughout  it.  A  is  the  pioneer  station,  that  of  the  San 
Antonio  Electric  Light  and  Power  Co.,  in  San  Antonio  Canon. 
It  was  started  in  1892,  a  single  phase  plant  at  6o~,  with  one 
120  KW  generator,  and  voltage  stepped  up  to  10,000  for  a  16- 
mile  transmission  to  Pomona  and  a  28-mile  line  to  San 
Bernardino.  This  plant  may  fairly  be  said  to  have  demon- 
strated the  commercial  feasibility  of  long  io,ooo-volt  lines. 
A  second  similar  generator  was  installed  later,  and  the  plant 
has  been  working  steadily  away  for  nine  years  with  singularly 
little  trouble,  in  spite  of  the  early  lack  of  experience  in  insu- 
lation. 

When  it  was  started,  plant  C,  the  second  of  the  group,  was  the 
pioneer  three-phase  transmission  on  this  side  of  the  Atlantic, 
and  was  installed  by  the  writer  as  an  8-mile  transmission  at 
2,500  volts.  It  contains  three  250  KW,  2500  volt  generators, 
two  of  which  were  in  the  original  installation  in  1893,  an(3 
raising  transformers  for  2,500  to  10,000  volts  to  the  amount  of 
about  600  KW.  These  were  added  later  for  the  line  to  River- 
side, about  18  miles  long.  A  second  station,  marked  B  on  the 
map,  was  built  later  3  miles  further  up  Mill  Creek  Canon,  in 
which  are  installed  two  revolving  field  250  KW,  11,500  volt 
three-phase  generators.  The  two  stations  are  worked  together, 
and  supply  Redlands,  Riverside,  and  Colton,  besides  smaller 
places.  At  Redlands  is  a  sub-station,  H^  with  750  KW  in 
transformers.  At  Riverside  is  another  sub-station,  E,  with 


6 10  ELECTRIC   TRANSMISSION  OF  POWER. 

375  KW  in  transformers  and  a  100  KW  rotary  converter;  at 
Colton  a  small  bank  of  120  KW  in  transformers  marked  Q. 
San  Bernardino,  at  Z),  has  a  small  sub-station  of  the  San 
Antonio  plant,  and  is  also  supplied  from  the  station  F  of  the 
San  Bernardino  Electric  Co.,  located  on  the  Riverside  ditch, 
where  a  fall  of  forty  feet  is  available,  and  containing  one 
1 20  KW  generator.  This  plant  is  a  60  cycle  one,  like  A,  the 
frequency  of  the  remaining  lines  shown  being  50^,  like  the 
original  Redlands  plant. 

At  G,  in  the  Santa  Ana  Canon,  was  established  the  great 
plant  of  the  Southern  California  Eltctric  Co.  already  men- 
tioned, which  was  started  in  1899.  This  power  station  is  oper- 
ated under  728  feet  head,  and  contains  four  750  KW  revolving 
fijld,  750  volt  generators  directly  coupled  to  Pelton  wheels 
at  300  r.  p.  m.  The  station,  of  which  the  interior  is  shown 
in  Plate  XX,  is  somewhat  out  of  the  ordinary  in  that  the 
wheels  and  generators  are  in  a  single  line,  the  wheels  being  in 
water-tight  steel  cases  and  the  tail  races  extending  across  and 
under  the  building,  an  arrangement  due  to  local  requirements. 
The  transformer  equipment  consists  of  twelve  250  KW  air 
blast  units  raising  the  line  voltage  to  33,000. 

The  main  line  extends  80  miles  to  Los  Angeles.  It  runs 
down  into  and  through  Redlands,  and  then  parallels  the 
Southern  Pacific  Railway  into  Los  Angeles.  On  the  way  at 
Pomona  is  the  sub-station  /,  with  three  150  KWoil  transformers 
and  a  so-light  constant  current  transformer  for  arcs.  About 
15  miles  further  on  near  Puente  is  a  little  sub-station  with 
three  150  KWoil  transformers  reducing  the  voltage  to  u,ooofor 
a  side  line  to  Anaheim  and  Santa  Ana,  at  which  latter  place  is 
a  little  sub-station  P,  with  225  KW  in  transformers.  In  Los 
Angeles  the  line  reaches  two  sub-stations,  N  and  O.  The 
latter  is  the  main  reducing  station,  where  six  750  KW  air  blast 
transformers  reduce  the  voltage  to  2,200  for  the  city  lines,  and 
for  transmutation  to  continuous  current.  This  is  accom- 
plished both  for  railway  and  power  purposes  by  motor  genera- 
tor sets.  These  were  chosen  in  lieu  of  rotary  converters  on 
account  of  the  greater  independence  of  the  continuous  current 
service  of  any  disturbances  on  the  alternating  current  side. 
The  motor  generators  are  combinations  of  synchronous  motor 
with  generator,  or  with  two  generators.  In  O,  the  lighting  sub- 


PLATE  XX. 


HIGH-VOLTAGE    TRANSMISSION.  6ll 

station,  are  500  KW  units;  in  N,  the  railway  sub-station,  300 
KW  units.  In  the  former  is  also  a  240  KW  synchronous  motor 
for  driving  other  generators.  This  whole  system  is  for  the 
work  of  the  Los  Angeles  Edison  Electric  Co.  Still  another 
section  of  the  33,000  volt  linesbranches  from  the  main  lines  five 
miles  south  of  Pasadena  and  reaches  the  sub-station  K  in  that 
city,  where  four  150  KW  oil  transformers  reduce  the  pressure 
to  2,200  volts  for  general  distribution  and  for  transmission  to 
Z,  the  power  station  for  the  Mt.  Lowe  Electric  Railway, 
where  a  150  HP  induction  motor  drives  the  railway  generators. 

Besides  the  extensive  system  just  described,  Los  Angeles  is 
reached  by  the  16,500  volt  line  of  the  San  Gabriel  Electric 
Co.  /,  the  power  station,  is  located  at  the  mouth  of  the  San 
Gabriel  Canon,  where  a  fall  of  400  feet  is  available.  The  gen- 
erators are  four  300  KW  two-phase  machines  at  50^,  the  line 
being  worked  three-phase,  as  is  the  usual  custom.  The  sub- 
station M  in  Los  Angeles  contains  a  full  set  of  reducing  trans- 
formers, in  units  of  125  KWand  above,  and  most  of  the  output 
is  delivered  to  rotary  converters  and  used  for  railway  purposes. 

All  these  systems  are  in  full  operation  and  well  loaded.  Save 
in  the  case  of  the  San  Antonio  line,  they  are  of  uniform 
periodicity,  5o~,  and  in  general  operate  their  distributing 
circuits  at  2,200  to  2,400  volts.  It  is  interesting  to  note  the 
successful  use  of  rotary  converters  at  this  periodicity  in  units 
as  large  as  400  KW. 

Altogether  the  high-tension  systems  aggregated  in  this 
region  form  one  of  the  most  remarkable  developments  of  elec- 
trical power  transmission  yet  accomplished. 

It  is  only  surpassed  by  the  group  of  transmissions  in  the 
neighborhood  of  Sacramento  and  San  Francisco,  which  is  of 
somewhat  later  growth,  and  not  even  now  in  full  operation — 
that  system  of  which  the  Standard  Electric  Co.  and  the  Bay 
Counties  Power  Co.  are  the  most  sensational  factors. 

Before  passing  from  this  region  mention  should  be  made  of 
the  remarkable  steam-driven  transmission  system  of  the 
United  Electric,  Gas,  and  Power  Co.  of  Santa  Monica,  which 
supplies  current  to  that  famous  resort,  and  also  transmits  power 
at  22,000  volts  to  Redondo,  San  Pedro,  Terminal  Island,  and 
Long  Beach.  This  is  one  of  the  very  few  long  distance  power 
transmissions  in  the  world  from  steam  plants.  The  only 


612  ELECTRIC    TRANSMISSION  OF  POWER. 

other  American  instance  of  any  importance  is  a  600  KW 
transmission  from  Bland,  N.  M.,  for  the  Cochiti  Gold  Mining 
Co.  This  has  a  3i-mile  line  at  17,300  volts. 

The  Santa  Monica  plant  has  a  400  KW  and  a  200  KW  three- 
phase  revolving  field  generator  at  6o~,  2,200  volts.  The  fuel 
used  is  a  thick  asphaltic  crude  oil  obtained  near  by.  The 
distribution,  being  almost  entirely  for  lighting,  is  essentially 
single  phase,  in  three  circuits  at  2,200  volts  in  Santa  Monica, 
while  three  100  KW  oil  transformers  raise  the  pressure  to  22,000 
for  the  three-phase  transmission.  Each  sub-station  has  three 
50  KW  oil  transformers  reducing  the  voltage  to  2,200  again, 
and  the  distributions  are  practically  single  phase,  as  at  the  main 
plant.  The  greatest  distance  of  transmission  is  35  miles. 

It  may  be  said  of  all  these  Californian  plants  that  they  have 
uniformly  worked  admirably,  have  encountered  few  unfore- 
seen difficulties,  and  have  as  a  rule  met  with  commercial 
success. 

Plants  working  at  10,000  and  15,000  volts  are  now  quite 
common,  and  are  uniformly  successful,  even  in  the  worst 
climate  the  United  States  can  furnish.  There  are  several 
10,000  volt  plants  which  have  been  working  admirably  for  sev- 
eral years  in  New  England,  where  the  conditions  in  winter  are 
notoriously  of  the  very  worst — and  others  in  situations  nearly 
as  unfavorable. 

Speaking  generally,  line  troubles  have  been  neither  frequent 
nor  annoying.  Insulators  have  seldom  failed  from  electrical 
causes,  and  accidents  to  apparatus  have  been  conspicuously 
rare.  It  is  not  too  much  to  say  that  the  experience  of  the  past 
few  years  has  shown  conclusively  that  such  voltages  as  10,000 
to  15,000  are  entirely  satisfactory  for  power  transmission,  even 
in  the  most  trying  climates  and  in  cases  where  complete 
reliability  is  of  the  first  importance. 

An  innovation  which  has  found  some  favor  is  the  use  of  gener- 
ators wound  to  deliver  directly  10,000  to  13,000  volts.  Such 
machines  are  always  built  with  stationary  armatures  and  revolv- 
ing fields.  Prior  to  the  introduction  of  this  construction,  gener- 
ators for  such  pressures  were  out  of  the  question,  but  there  are 
now  in  use  a  considerable  number  of  these  high-voltage  ma- 
chines, nearly  all  above  500  KW  capacity,  which  are  uniformly 
doing  good  service.  The  ultimate  economy  of  this  practice  is  not 


HIGH-VOLTAGE  TRANSMISSION.  613- 

yet  clear.  One  set  of  transformers  is  saved,  reducing  the  loss 
about  2.5  per  cent.,  while  on  the  other  hand  the  very  large 
insulation  space  required  tends  considerably  to  reduce  the 
capacity  of  generators  of  given  dimensions.  Exact  figures 
cannot  be  had,  since  the  intrinsic  merits  of  the  matter  are  mixed 
up  with  commercial  considerations  involving  the  competitive 
and  advertising  value  of  novelty,  the  question  of  adaptation 
of  standard  sizes  and  speeds,  and  other  matters  which  are 
of  temporary  rather  than  permanent  importance.  It  is  the 
author's  impression,  however,  that  when  these  matters  are 
eliminated  there  will  prove  to  be  no  very  great  difference  in  cost, 
including  depreciation  and  efficiency,  between  the  high-voltage 
machine  and  a  low-voltage  one  plus  the  raising  transformers. 
At  present,  inasmuch  as  the  high-voltage  generators  are  excel- 
lent and  reliable  machines, their  use  is  advantageous  so  long  as 
a  good  bargain  can  be  driven  as  to  price  as  compared  with  the 
low-voltage  machine  and  its  raising  transformers.  Although 
none  of  these  high-voltage  generators  have  been  long  enough 
in  use  to  get  a  final  idea  of  their  rate  of  depreciation,  there  is 
not  much  to  be  feared  in  view  of  our  present  knowledge  of 
insulation. 

The  phenomena  of  line  capacity  and  inductance,  resonance, 
unbalancing  of  three-phase  transmission  circuits,  and  divers 
other  unpleasant  things  which  are  theoretically  present  on 
long-distance  or  high-voltage  lines,  have  been  shown  by  experi- 
ence to  be  of  no  sensible  account  in  a  well  designed  and  con- 
structed plant.  Line  inductance  and  the  rest  are,  of  course, 
always  with  us,  but  from  an  operative  standpoint  they  are  not 
at  all  serious  at  any  pressure  or  distances  now  in  use.  They 
must  be  considered  and  taken  into  account  just  like  ohmic  re- 
sistance and  line  insulation,  but  are  not  material  obstacles  at 
any  voltage  or  distance  yet  tried.  On  pages  622,  623,  and 
624  is  a  pretty  complete  list  of  all  the  American  plants  using 
regularly  pressures  of  10,000  volts  or  more. 

It  is  a  rather  formidable  list,  and  the^  number  of  plants 
now  operating  at  or  even  above  20,000  volts  is  somewhat 
startling.  Everything  now  indicates  that  pressures  of  such 
magnitude  are  likely  to  be  regularly  used  in  the  future,  in 
cases  where  extreme  measures  may  seem  to  be  necessary. 

This  list  is  rapidly  extending  under  the  encouragement  of 


614  ELECTRIC    TRANSMISSION  OF  POWER. 

experience  and  the  goad  of  the  present  very  high  price  of 
copper.  So  soon  as  the  cost  of  copper  for  the  highest  voltage 
convenient  for  general  distribution,  say  2,500  volts,  gets  at  all 
burdensome  the  tendency  is  to  pass  at  once  to  10,000  volts  or 
more  for  the  transmission.  Voltages  of  5,000  or  6,000,  counted 
high  a  few  years  since,  now  hardly  come  in  for  serious  consid- 
eration. The  main  points  of  high-voltage  work  seem  now 
well  settled,  but  there  is  still  much  room  for  experiment  and 
improvement  in  the  details  of  the  system.  With  rare  excep- 
tions the  transmission  lines  are  worked  three-phase,  either 
from  three-phase  generators,  or  from  two-phase  generators 
the  change  to  three-phase  being  accomplished  in  the  trans- 
formers as  described  on  p.  182. 

As  regards  the  generation  and  distribution,  the  work  in  this 
country  is  about  equally  divided  between  the  two-phase  and 
three-phase  systems,  the  latter  being  rather  in  the  majority. 
As  might  be  anticipated,  there  is  found  very  little  difference  in 
the  general  properties  of  the  two  systems,  the  tendency  being 
to  use  two-phase  in  many  cases  where  an  existing  alternating 
lighting  system  is  to  be  supplied,  and  three-phase  for  heavy 
motor  work  or  when  the  whole  system  is  installed  de  novo, 
although  neither  this  nor  any  other  plan  is  consistently 
adhered  to  in  practice. 

The  generating  and  transforming  units  have  been  steadily 
increasing  in  voltage  and  capacity.  Some  2,^00  KW  trans- 
formers have  recently  been  installed  at  the  Niagara  plant, 
while  those  of  several  hundred  kilowatts  are  common  enough. 
Barring  the  general  objection  to  putting  too  many  eggs  in  one 
basket,  this  tendency  is  a  good  one,  giving  units  at  once 
cheaper  and  more  efficient  than  the  smaller  sizes. 

As  regards  the  line  itself,  apart  from  the  use  of  special 
insulators  and  widely  spaced  lines  as  already  noted,  there  is 
little  of  a  striking  character  to  chronicle.  The  tendency  is 
steadily  toward  the  use  of  extra  strong  poles,  35'  to  40'  long, 
and  not  less  than  8"  in  diameter  at  the  top.  With  our  present 
command  of  high  voltages  it  is  seldom  that  the  line  wire  is 
larger  than  No.  i,  duplicate  circuits  being  used  if  the  amount 
of  energy  transmitted  is  greater  than  could  properly  be  sent 
over  No.  i  wire. 

The  use  of  aluminium  wire  to  some   extent  has  created  a 


HIGH- VOLT  AGE    TRANSMISSION.  615 

tendency  to  use  longer  spans  in  the  line  than  are  given  by  the 
usual  50  poles  per  mile.  Some  lines  have  been  tried  with  as 
low  as  25  to  30  poles  per  mile,  but  in  view  of  the  mechanical 
properties  of  aluminium  wire,  this  seems  to  the  writer  a  step 
in  the  wrong  direction.  It  certainly  would  not  do  for  lines 
exposed  to  sleet  or  heavy  winds,  for  under  these  circum- 
stances the  greater  bulk  of  the  aluminium  conductor  is  a 
menace. 

Cross-arms  are  not  infrequently  treated  with  insulating 
compounds  in  the  manner  already  suggested.  The  material 
used  is  generally  some  asphaltum  compound  applied  hot,  or 
linseed  oil  well  boiled  in. 

Lines  are  usually  transposed  at  intervals  of  from  half  a  mile 
to  a  mile,  in  cyclic  order.  In  some  instances  this  is  done  in  a 
span  of  half  the  usual  length  to  lessen  the  strain. 

It  is  curious  to  note  that  aside  from  things  falling  or  thrown* 
across  the  line,  large  birds  like  hawks,  eagles,  and  owls  are  the 
commonest  cause  of  short  circuits.  Most  of  the  long  lines 
have  suffered  from  this  cause,  some  of  them  rather  frequently. 
Wide  spacing  of  the  lines  is  about  the  only  remedy  for  this 
particular  annoyance.  A  line  spaced  as  shown  in  Fig.  278,  for 
instance,  could  hardly  be  crossed  unless  by  the  largest  eagles, 
and  is  unlikely  to  be  more  than  momentarily  crossed  by^ 
anything  lodged  upon  it  by  accident  or  design.  The  usual 
arrangement  of  the  wires  in  triangle,  with  the  vertex  up,  is 
commendable  as  averting  easy  lodgment  of  anything  across 
the  line,  if  for  no  better  reason.  Accessory  apparatus  has 
hardly  kept  pace  with  the  general  progress  of  the  art.  Switch- 
board appliances  for  high-voltage  work  notably  are  unde- 
veloped into  standard  forms.  They  have  particularly  suffered 
from  the  too  common  straining  after  compactness.  A  liberal 
factor  of  safety  is  always  a  good  thing,  and  nowhere  is  it  more 
necessary  than  in  dealing  with  voltages  such  as  are  used  in 
electrical  power  transmission. 

There  is  at  present  a  strong  tendency  to  let  the  high  tension 
lines  alone,  so  far  as  is  possible,  arranging  switches,  fuses, 
cut-outs,  and  the  like  on  the  low  voltage  side  of  the  trans- 
formers. In  many  instances  the  transmission  line  proper 
is  entirely  without  such  devices.  To  tell  the  truth  there  is 
little  need  of  them  in  simple  transmissions,  although  in  more 


616  ELECTRIC   TRANSMISSION  OF  POWER. 

complex  cases,  with  several  line  circuits,  they  may  often  have 
to  be  used. 

When  high  tension  switches  have  to  be  used,  enough  experi- 
ence has  now  accumulated  to  indicate  that  it  is  always  wise  to 
use  an  oil-break  switch.  Not  only  is  the  danger  of  arcing 
very  greatly  reduced,  but  there  is  far  less  chance  of  producing 
unpleasant  resonance  effects  on  the  line  than  if  an  air-break 
switch  is  used.  In  the  latter  case  it  is  possible  not  only  to 
draw  an  arc  of  prodigious  length,  but  to  throw  a  resonant 
voltage  upon  the  line  sufficient  to  break  down  insulation  at 
vital  points.  In  some  recent  experiments  at  40,000  volts  it 
was  possible  to  draw  an  arc  nearly  thirty  feet  in  total  length 
from  a  quick-break,  long-armed  air  switch,  and  breaking  the 
circuit  in  this  manner  gave  surging  electromotive  forces  in  the 
line  two  or  three  times  the  high  normal  voltage  of  the  system. 
With  an  oil-break  switch,  having  its  contacts  in  separate  oil 
receptacles,  currents  at  40,000  volts,  representing  more  than 
1000  KW,  could  be  opened  instantly  and  certainly,  without  a 
flash  or  any  surging  rise  of  potential. 

On  account  of  the  danger  of  such  surging,  it  seems  advisable 
to  use  oil-break  switches  for  all  very  high  voltage  work,  and 
all  work  of  considerable  magnitude  at  ordinary  primary 
voltages. 

In  controlling  very  large  amounts  of  energy,  several  thou- 
sand KWand  upward,  it  is  a  difficult  matter  to  install  manually 
operated  switches  in  any  position  at  once  safe  and  accessible. 
The  consequences  of  a  heavy  arc  at  high  voltage,  as  in  open- 
ing a  short  circuit,  are  so  terrific  that  switches  liable  to  such 
service  are  now  very  often  arranged  to  be  worked  by  pneu- 
matic cylinders  or  by  electric  motors  controlled  from  a 
distance.  By  this  device  the  switches  can  be  far  removed 
from  danger  to  persons  or  property,  and  may  be  given  ample 
space  for  safety,  without  making  a  switchboard  too  large  for 
convenient  operation.  Perhaps  the  most  complete  develop- 
ment of  this  idea  is  in  the  new  Niagara  plant,  in  which  the 
controlling  switches  are  placed  on  a  sort  of  dummy  switch- 
board on  a  small  scale,  on  which  each  control  switch  puts  into 
action  the  corresponding  real  switch  wherever  it  may  be  in 
the  station. 

Fig.    283    shows   a   recent   electrically    operated    oil-break 


HIGH-  VOL  TA  GE   TRA  NSMISSION. 


617 


switch  for  a  12,000  volt,  three-phase  circuit  carrying  several 
hundred  amperes  per  branch.  The  essential  features  are 
obvious.  A  U-shaped  plunger  breaking  contact  under  oil  at 
each  end  serves  to  break  each  phase  at  two  points,  the  ends 


FIG.  283. 

being  in  separate  oil  tanks.  The  three  plungers  are  worked 
from  a  common  cross-head  actuated  by  the  motor.  Switch- 
board appliances  for  heavy  transmission  work  form  a  branch 
of  engineering  of  no  mean  importance.  Experience  on  the 
subject  is  rapidly  accumulating,  but  almost  every  large  plant 
so  far  has  been  treated  by  special  designs.  The  controlling 
elements  in  design  should  be  accessibility  and  simplicity. 
Switches,  cut-outs,  and  the  like  should  be  planned  to  do  the 
necessary  work  of  the  system  well,  and  no  more  than  is  really 
necessary. 

It  may  not  be  out  of  place  here  to  refer  to  the  Nernst  lamp, 
which  has  just  been  brought  out  in  this  country,  being  already 
somewhat  used  abroad.  The  principle  of  this  lamp,  as  is  well 


618  ELECTRIC   TRANSMISSION  OF  POWER. 

known,  is  the  raising  to  incandescence  of  a  pencil  of  refrac- 
tory material  by  the  passage  of  electric  current.  The  pencil 
or  "glower"  is  of  a  material  similar  to  that  used  in  Wels- 
bach  mantels,  and  is  a  non-conductor  when  cold,  but  a  fairly 
good  conductor  at  high  temperatures,  say  900°  to  1000°  C. 
The  conduction  is  then  accompanied  by  electrolytic  action, 
so  that  the  glower  is  rather  short-lived  when  used  with 
continuous  current.  As  the  glower  alone  is  hypersensitive  to 
changes  of  voltage  when  hot,  it  is  used  in  series  with  a 
"ballast"  resistance  of  iron  wire  sealed  in  an  oxygen-free 


FIG.  284. 

tube  to  preserve  it.  The  high  temperature  coefficient  of  the 
iron  steadies  the  glower. 

The  initial  temperature  necessary  for  conduction  in  the 
glower  is  obtained  by  a  "  heater"  consisting  of  platinum  wire 
enameled  upon  a  porcelain  core  and  located  just  above  the 
glower.  This  heater  is  in  shunt  to  glower  and  ballast,  and  is 
automatically  cut  out  by  a  little  magnetic  break  when  the 
glower  is  at  work.  The  heater  will  bring  the  glower  into 
action  in  about  half  a  minute. 

The  heaters  and  glowers  are  enclosed  in  a  globe  to  keep  in 
the  heat  and  keep  out  draughts,  and  the  works  other  than 
these  are  in  a  small  cap  above  the  globe.  Fig.  284  shows  the 
glowers  and  heaters  of  the  six-,  three-,  and  one-glower  lamp 
assembled  on  their  base  with  the  projecting  pins  ready  to 
make  automatic  connections  when  shoved  into  place  in  the 


HIGH-  VOL  TA  GE    TRANSMISSION. 


619 


lamp;  and  Fig.  285  shows  the  six-glower  lamp  as  fitted  for  out- 
door use.  Each  glower  is  about  0.04"  in  diameter  and  i'  long, 
and  is  good  for  approximately  50  c.  p.  normal  to  its  axis; 


FIG.  285. 

a  little  more  in  the  multiple  glower  lamps  where  the  tempera- 
ture is  carried  well  up. 

The  globes  require  cleaning  about  every  100  hours  from 
a  deposit  off  the  glowers,  while  the  average  life  of  a  glower  is 
stated  to  be  nearly  800  hours.  The  efficiency  of  the  Nernst 
lamp  as  developed  in  this  country  is  somewhat  better  than  that 
of  a  high  efficiency  incandescent  lamp,  and  about  the  same  as 
that  of  an  enclosed  arc.  Its  light  is  beautifully  white  and 


620  ELECTRIC   TRANSMISSION  OF  POWER. 

steady  and  admirably  suited  for  commercial  use.  Reports 
from  tests  indicate  a  considerable  field  for  usefulness  for  the 
Nernst  lamp  in  -power  transmission  plants,  especially  in  replac- 
ing the  constant  potential  alternating  arcs,  which  are  always 
somewhat  troublesome  in  the  matter  of  noise.  As  a  com- 
petitor of  the  incandescent  lamp  it  has  the  advantage  of  better 
color  and  efficiency,  but  requires  care  like  an  arc  lamp. 

It  is  too  early  yet  to  properly  evaluate  the  place  of  the 
Nernst  lamp  in  the  art,  but  it  is  certainly  promising. 

The  question  of  frequency  is  gradually  settling  itself. 
Except  in  plants  intended  mainly  or  entirely  for  use  with 
rotary  converters,  a  frequency  of  about  6o~  is  the  general 
rule,  and  this  is  the  figure  adopted  in  the  great  majority  of 
transmission  plants.  In  those  intended  for  general  distribu- 
tion of  power  and  light,  the  lighting  sets  a  lower  limit  to  the 
frequency  that  must  be  respected.  In  the  neighborhood  of 
3o~  incandescent  lighting  becomes  decidedly  troublesome, 
and  if  alternating  arcs  are  to  be  used  the  frequency  must  be 
kept  above  4o~.  On  the  other  hand,  induction  motors  give 
the  best  results  at  moderate  frequency,  say  not  over  50-60^ 
at  the  most.  All  these  facts  point  to  the  advisability  of  keep- 
ing within  moderate  limits,  and  the  usual  6o~  is  for  ordinary 
distances  and  ordinary  distributions  very  satisfactory.  In  the 
writer's  judgment  it  is  rather  high  for  heavy  long-distance 
WOrk — 45  or  50^  would  be  rather  better,  but  there  is  no  occa- 
sion whatever  for  going  as  low  as  30^,  except  when  rotary 
converters  are  the  principal  load  and  no  lighting  whatever  is 
to  be  done  from  the  transmission  lines. 

In  this  connection  it  is  well  again  to  call  attention  to  the  fact 
•that  alternating  arc  lamps,  particularly  of  the  inclosed  type, 
are  now  in  a  fairly  satisfactory  state,  and  that  the  series  arc  is 
being  very  largely  replaced  by  constant-potential  arcs,  often 
inclosed,  working  off  the  ordinary  lighting  system.  It  there- 
fore appears  probable  that  special  devices  for  arc  lighting  will 
form  a  less  troublesome  feature  of  transmission  work  than 
hitherto. 

The  induction  motor  has  come  into  wider  and  wider  use  as 
transmission  plants  have  increased  in  number,  and  particularly 
the  larger  sizes  have  been  well  developed.  A  good  example 
of  this  tendency  is  shown  in  Plate  XXI,  Fig.  2,  a  500  HP 


PLATE  XXI. 


HIGH-VOLTAGE    TRANSMISSION.  621 

Westinghouse  induction  motor  recently  installed.  These  very 
large  induction  motors  are  particularly  valuable  for  hard  ser- 
vice and  starting  with  considerable  loads,  although  in  view  of 
the  value  of  synchronous  motors  in  improving  the  power 
factor  of  transmission  plants,  these  latter  machines  are  not 
likely  to  be  entirely  superseded. 

It  is  to  be  hoped  that  in  the  future  manufacturers  will  pay 
closer  attention  to  the  design  of  induction  motors,  especially 
in  the  direction  of  getting  uniformly  good  power  factors. 
While  the  best  of  the  induction  motors  leave  little  to  be  desired 
in  this  respect,  there  are  many  motors  in  service  having  unde- 
niably bad  power  factors.  This  is  due  partly  to  efforts  to 
cheapen  construction  and  partly  to  a  demand  for  motors  of 
special  sizes  and  unusual  speeds.  This  is  a  thing  which  should 
be  taken  under  careful  consideration  by  those  operating  trans- 
mission plants,  who  are  the  chief  sufferers.  While  it  may  not 
be  advisable  to  sell  power  for  induction  motors  by  current 
meter  rather  than  by  watt  meter,  the  demand  for  current 
should  certainly  be  taken  into  account.  If  the  user  of  a 
motor  realizes  that  to  keep  down  the  cost  of  power  it  is  best 
to  get  the  best  motor  attainable  and  deny  himself  the  free 
scope  of  his  fancy  for  engineering  freaks,  the  present  trouble 
will  soon  be  suppressed.  Small  motors  for  very  low  speeds 
and  motors  without  proper  starting  appliances  are  responsible 
for  most  of  the  difficulties  now  encountered. 

In  general,  the  results  of  the  recent  experience  in  transmis- 
sion work  have  been  to  confirm  the  hopes  of  a  few  years  ago 
and  to  give  a  bright  outlook  for  the  future.  The  high-voltage 
question  has  been  definitely  settled  in  the  affirmative,  and  with 
it  the  possibility  of  commercial  transmission  of  power  over 
longer  distances  than  have  heretofore  been  judged  practicable. 
Many  details  remain  yet  to  be  perfected,  but  if  experience 
counts  for  anything  these  will  be  attended  to  in  due  season. 

As  a  class  the  plants  using  10,000  volts  or  more  have  given 
singularly  little  trouble,  and  have  been  remarkably  free  from 
accidents  of  all  sorts. 

In  Plate  XXI.  are  shown  the  two  typically  modern  pieces  of 
power  transmission  machinery — one  the  500  HP  induction 
motor  of  Westinghouse  make  just  referred  to,  the  other  a 
12,000  volt  Stanley  two-phase  generator  with  stationary  ar- 


RKMARKS. 

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George  H  .  Gow  
Lowell  Water,  Lt.  &  Power  Co  
Tuolumne  Co.  Elec.  Pr.  &  Lt.  Co.. 
Big  Creek  Power  Co  
San  Miguel  Con.  Gold  Mining  Co. 
Central  California  Elec.  Co  

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G.  &  O.  Braniff&  Co  
Cia  Electrica  &  Irrigadora 

Portezuelo  Light  &  Power  C 

HIGH-VOLTAGE    TRANSMISSION.  625 

mature,  photographed  while  in  operation.  Aside  from  high- 
voltage  generators  and  very  large  induction  motors  there  is  as 
yet  little  that  is  novel  in  the  way  of  heavy  power  transmission 
machinery. 

Altogether,  the  past  few  years  have  been  wonderfully  pro- 
ductive of  results,  if  not  of  methods,  in  the  electrical  trans- 
mission of  power,  and  the  methods  employed  are  so  far  past 
the  experimental  stage  that  the  engineer  need  not  be  stag- 
gered at  any  problem  likely  to  be  met  for  some  time  to  come. 


INDEX. 


Accumulators,  hydraulic,  44 
Air,  compressed,  friction  of,  51 
Air,  compressors,  48,  49,  50 

compressors,  efficiency  of, 

50 

hydraulic,  53,  54,  55,  56 

Alternating  arc  lamps,  510 
Alternating  arcs,    constant    cur- 
rent transformers  for,  511,  512, 

5i3 

-  ,   constant  current  regula- 
tors for,  513,  514 

Alternators,   armature    reactions 

in,  161 

,  compounding  of,  170,  171 

,  design  of,  155,  156 

,  inductance  of,  160,  161 

,  inductor,  186,  187 

,  polyphase,  172 

,  practical  arrangement  of, 

185,  186,  187 
,  regulation  of ,  168,  169,  170, 

188 
,  windings  of,  157,  158,  159, 

1 60 

with  revolving  field  mag- 
nets, 187,  188 
Aluminum,   for    overhead    lines, 

432,  433 

Ammeters,  575,  576,  577 
Ampere,  20 
Anchor  ice,  381 
Armature  reaction,  450 

windings,  drum,  76,  77 

windings,  gramme,  78,  79, 

80 
Auxiliary  plants,  554,  555,  556 

Batteries,  storage,  28,  553,  554 
Biberest,  power  transmission  at, 

"4.  "5 

Boilers,  classification  of,  305,  306 

,  efficiency  of,  307,  308 

,  efficiency  of,  tests  of,  308, 

309 

,  forcing  output  of,  307 

,  practical  efficiency  of,  310 


Capacity  and  inductance  in  paral- 
lel, 147 

and  inductance  in  series,  146 

•  and    inductance,  relations 

between,  142,  143 

,  electrostatic,  139 

,    electrostatic,     equivalent 

in  current,  141 

electrostatic,  of  practical 


apparatus,  146,  455, 

,  electrostatic,  unit  of,  140. 


-  impedance,  141 
--  of  lines,  455 

Catenary    curve,    properties    of, 

47i 

Circuits,  inductive  energy  in,  133 
Commutator,  principle  of,  17 
Commutators,  rectifying,  267,  268 

-  .rectifying,  advantages  and 
disadvantages  of,  273.  274 

-  ,  rectifying  difficulties  met 
in,  268,  269 

Condensance,  141 
Condenser,  electrostatic,  140 
Counter  electromotive  force,  85 

-  E.   M.    F.,    relation   of,  to 
output  and  efficiency,  86 

Cross  arms,  478,  479 
Currents,   alternating,   classifica- 
tion of.  154 

-  ,  alternating,  generation  of, 
123,  124 

--  ,  continuous,  advantages  of  , 

118,  119 
Current,     continuous,     armature 

windings.for,  76,  77,  78 

-  ,  continuous  production  of, 


75 


-,  electrical,  propagation  of,  7 
induction   of,  in  moving 


conductors,  14 

,  leading,  144 

.   relation   of.   to    electrical 


charge,  10 

reorganizes,  classes  of,  266 

,  unit  of,  20 


627 


628 


INDEX. 


Dams,  374 

,  masonry,  374,  375 

,  timber  crib,  376,  377 

Distribution  by  secondary  mains, 
506,  507,  508 

,  conditions  of,  500,  501 

,    diaphase   two   wire,    529, 


530 
520 


from  eccentric  stations,  519, 


,  house  to  house,  efficiency 

of,  503,  504,  505,  506 

,  kinds  of,  499 

,  linear,  521,  522,  523 

,  monocyclic,  528,  529 

,     monophase  ,  three-wire, 

527,  528,  453 
monophase  two-wire,  526 

of  power,  electric,  65,  66 

,  radial,  516,  517,  518,  519 

,  sub-station,  524,  525 

,    sub-station,    example   of, 

540,  541,  542,  543 

,  sub-station,  secondary  volt- 
ages for,  525,  526 

,  three-phase,  532,  533 

,  three-phase  four-wire,  533 

,  three-phase,  modifications 

of,  534.  535 
Draft  tubes,  326,  333 
Dynamo,  continuous-current,  18 

,  principles  of,  16 

Dynamotors,  279,  280,  281 
,  advantages  and  disadvan- 
tages of,  280,  281 
Dyne,  20 

Electricity,  nature  of,  i 
Electrification,  8 

,  nature  of,  9 

Electromotive  forces,  composition 

of,  130 

force,  effective,  128 

force,  impressed,  127,  128 

,  force,  inductive,  127 

,  forces,  periodic,  124 

,  force,    periodic,     current 

produced  by,  126 
force,  unit,  derivation  of, 

20 
Energ3%  available  sources  of,  23 

C.  G.  S.  unit  of,  20 

kinds  of,  4 

nature  of,  2 

potential  and  kinetic,  2 

radiant,  5 

real  and  apparent,  132,  133 

tidal,  utilization  of,  26 

transformations  of,  3 


Energy,  transmission  of,  as  fuel, 
28 

Engines,  gas,  cost  of  power  from, 
559,  5^0 

,  hydraulic,  43 

,  pneumatic,  51 

,  solar,  26 

,  steam,  classification  of, 

292,  293 

,  steam,  coal  consumption 

of,  311,  312 

,  steam,  cost  of  power  from, 

558,  559 

,  steam,  economy  of  various 

types  of,  299,  300 

,  steam,  efficiency  of,  290, 

291 

,  steam,  for  direct  coupling, 

303,  304 

,  steam,  principles  of  econ- 
omy in,  291,  292 

,  steam,  steam  consumption 

of,  297,  298 

,  steam,  table  of  steam  con- 
sumption of,  299 

,  steam,  valve  gear  of,  293, 

294 

Engines,  steam,  variation  of  econ- 
omy of  with  load,  301,  302 

Exciters,  arrangement  of,  400 

Farad,  141 

Ferranti  constant-current  recti- 
fier, 271,  272 

Field,  magnetic,  n 

,  magnetic,  strengthening 

of,  by  iron,  15 

,  magnetic,  transference  of 

energy  by,  12 

windings,  compound,  83,  84 

winding,    differential,   96, 

97 

windings,  series,  82 

windings,  shunt,  82 

Floriston,  Cal.,  transmission  plant 

at,  405,  406,  407 
Folsom,  Cal.,  transmission  plant 

at,  400,  401,  402 
Force,  electromotive,  7 
Frequencies,  commercial,  264 
Frequency  meter,  581 
Fresno,  Cal.,  transmission  plant 

at,  403,  404,  405 
Fuel,  cost  of,  in  a  large  modern 

plant,  322,  323 

,  supply  of,  23 

oil,  use  of,  312,  313 

Fuels,    comparative    evaporative 

power  of,  308,  309 


INDEX. 


629 


Generators,  diphase,  of  Niagara 
plant,  175,  176 

,  polyphase,  armature  re- 
actions in,  174 

,  polyphase  winding,  173, 174 

,  triphase,  177 

Genoa,  power  transmission,  by 
constant  current  at,  102,  103,  104 

Governors,  hydraulic,  differential, 
348,  358,  359 

,  hydraulic,  actual  perform- 
ance of,  352,  353.  357. 

,  hydraulic,  classification  of, 

346,  347,  348 

,  hydraulic,  types  of,  350, 

351,  352,  353,  354,  355,  35&,  357, 
358,  359 

Governing,  hydraulic,  conditions 
affecting.  359,  360 

,  hydraulic,  difficulties  in, 

345,  346 

Guy  ropes,  480 

Harmonics  in  alternating  current 

waves,  165,  166 

and  resonance,  167 

Henry,  135 

Heterophase  systems,  183,  184, 185 

High     voltage    circuits,     danger 

from,  443,  444 

voltage,  experience   with, 

438,  439 

voltage,  experiments  with, 

440,  594,  595,  596 

voltage  generators,  installa- 
tion of,  408 

voltage  line  construction, 

602,  603,  604,  605,  606,  607,  614, 
615 

voltage      generators      vs. 

transformation,  457,  458 
voltage,  limitations  of,  440, 

441 
voltage  lines  underground, 

445 

44  Hunting  "  of  synchronous  ma- 
chines, 224,  225,  226 

,  remedies  for,  227 

Hydraulic  pipe  lines,  378 

pipe    lines,   anchoring  of, 


379 
378 
380 


pipe  lines,  construction  of, 
pipe,  table  of  dimensions  of, 


Impedance  factor,  application  of, 

449,  450 

factors,  table  of,  448 

Impedances  in  parallel,  138 

in  series,  138,  139 

Impulse  wheels,  337 
Incandescent  lamps,  high  voltage, 

524,  525 
Inductance,  128 

,  determination  of,  134,  135 

of  circuits,  447 

of  lines,  455 

of  lines,  practical  effect  of, 

448 
,  practical  examples  of,  136, 

production  of  lag  by,  128 

,  regulation  by,  153 

,  unit  of,  135 

Induction,  electromagnetic,  13 

motors,  armature  speed  in, 

232,  234 

motors.fundamental  princi- 
ples of,  229,  230,  231 

motors,   general  construc- 


137 


tion  of,  237,  238,  239 
motors,    heterophase    and 

monophase,  247,   248,  249,  350, 

251,  252 
motors,  power  factors  of, 

253,  256,  257 
motors,  practical  properties 

of,  252,  253,   254,  255,   256,  257, 

258 
motors,  primary  windings 

of,  238,  239 

motors,     relation     of,     to 


motors  in  general,  228,  229 

motors,  rotary  field  theory 

of,  235,  236 

motors,    secondary    wind- 
ings of,  237,  242 

motors,    speed    regulation 

of,  261,  262,  263 

motors,  starting  torque  of, 

257,  258,  259,  260 

motors,  tests  of  commercial, 


Impedance,  131 

,  determination  of,  134,  135, 

136 


255,  256,  257,  258 
motors,  typical  construction 

of,  229 
motors,  weak  points  of,  252, 

253 
motors,   with    condensers, 

254,  256 
Inductive  circuits,  electromotive 

forces  in,  128 

Insulating  materials,  441,  442,  443 
Insulators,  design  of,  435 
,  materials  for,  436,  437 


630 


INDEX. 


Insulators,  oil,  437 
Insulator  pins,  482,  483 
Insulators,  special  high  voltage, 

597.  598,  599,  600,  601 
Insulation,    testing    of,    at    high 

voltage,  485 

Kelvin's  law  of  economy,  423 

law,  modifications  of,  424, 

425 

La  Chaux-de-Fonds,  constant  cur- 
rent transmission  at,  106,  107 

Lag,  angle  of,  128,  129 

,  relation  of,  to  energy,  132, 

133 

Lead,  angle  of,  143 

Lightning,  486,  487 

arresters,  487,  488,  489,  490, 

491,  492,  493 

arresters,  installation  of, 

493,  494,  495 

arresters,  with  inductance 

coils,  487,  488 

guard  lines,  494,  495 

Line,  losses  in  the,  418 

Line,  relation  of,  to  voltage  of 
transmission,  421,  422 

Lines,  conditions  governing  de- 
sign of,  422,  423,  424,  425,  426 

,  insulation  of,  434 

,  material  for,  431,  432,  433, 

434 

,  overhead  and  under- 
ground, 428,  429,  430 

List  of  American  plants  at  or 
above  10,000  volts,  622,  623,  624 

Mesh  connection  of  three-phase 

circuit,  1 80 

Meters,  induction,  584 
Microfarad,  141 
Miner's  inch,  365,  366 
Monocyclic  system,  183,  184 
Motor,  generator,  107 
generators,   274,    275,   276, 

277,  278 
generators,  advantages  and 

disadvantages  of,  276 
generators,  double- wound, 

279,  280,  281 

,  principles  of  electric,  17 

:  Motors,  distribution  of  power  by, 

65,66 

— ,  electric,  cost  of.  562 

,  electric,      efficiencies      of 

different  sizes  of,  61 

,  electric,  torque  in,  84 

•  '..  .',  hydraulics,  43 


Motors  on  lighting  circuits    515 
5i6 

-  ,  series,   at  interdependent 
current  and  voltage,  92,  93 

-  ,  series,    constant    current. 
87,  88 

-  ;  —  ,  series,    constant    current, 
disadvantages  of,  89 

-  ,  series,    constant    current, 
speed  regulation  of,  88 

,  series,  constant  potential, 


properties  of,  90,  91,  92 

-  ,  series,  constant  potential, 
regulation  of,  90,  91 

-  ,  shunt,  properties  of,  94 

-  ,  shunt,  speed  regulation  of, 
95,  96 

-  ,  special    methods    of    reg- 
ulating, 97,  98 

-  ,  wave,  29 

Niagara  station,  plan  of,  392 
Nernst  lamp,  617,  618,  619 

Ohm,  20 

Ohm's  law,  418,  419 
Oregon  City,  transmission  plant 
at,  388,  389,  390 

Parallel  running  of    alternators, 

409,  410,  411 

Pelton  wheel,  327,  328,  337 
Pipes,  hydraulic,  loss  of  head  in, 

46 

-  ,  pneumatic,  loss  of  head  in, 
5i 

Pole  line,  Niagara-Buffalo,  483,  484 

-  ,  magnetic,  unit,  20 
Poles,  proper  setting  of,  477,  479 

-  ,  stresses  upon,  479,  480 

-  ,  table    of    dimensions    of, 
477 

-  ,  wind  pressure  upon,  480, 
481 

Pollak     rectifying     commutator, 

270,  271 
Polyphase  circuits,   various  con- 

nections of,  200,  201,  202,  203 
Power,  determination   of  selling 

price  of,  569,  570,  571,  572,  573 

-  ,  electric,  cost  of,  on  very 
large  scale,  562,  563,  564 

-  factor,  136 

-  ,  general  cost  of,  560,  561 

stations,   arrangement   of, 


392,  393,  398,  399 
stations,  direct  coupling  in, 

394,  395,  397 

stations,  foundation  for,  392 


INDEX. 


631 


Power  station,  selection  of  site 
for,  388 

stations,  size  of  trans- 
former units  in,  411,  412 

stations,   size  of  units  in, 

338,  395,  39<>,  399 

transmission  at  constant 

current,  99,  100,  101,  102 

transmission  at  constant 

voltage,  109,  no,  in 

transmission  by  gas,  57 

transmission  by  interde- 
pendent dynamos  and  motors, 

112,   113 

transmission  by  ropes,  33 

transmission  by  ropes,  cost 

of,  68 

transmission  by  ropes,  effi- 
ciency of,  37 

transmission  by  ropes, 

examples  of,  38 

transmission  by  ropes, 

ropes  and  pulleys  for,  41 

transmission,  comparison 

of  methods  of,  68,  69,  70 

transmission,  electric,  57 

transmission,  electric,  cost 

of,  69 

transmission,  electric,  effi- 
ciency of,  59,  60,  6 1,  62 

— —  transmission,  electric,  na- 
ture of  losses  in,  58 

transmission      from      coal 


mines,  30 

transmission,  general  con- 
ditions of,  22 

transmission,  hydraulic,  42 

•  transmission,  hydraulic  effi- 


ciency of,  43,  44,  46' 
transmission,       hydraulic, 

examples  of,  42,  45 

transmission ,methods  of,  31 

transmission,  pneumatic,  47 

transmission,     pneumatic, 


cost  of,  68,  69 

transmission,     pneumatic, 

efficiency  of,  52 

transmission,       pneumatic 


reheated,  cost  of,  69,  70 

transmission,     pneumatic, 


with  re-heating,  52 

transmission  systems,  rela- 
tive efficiency  of,  72 

,  variation  of  cost  of,  with 

out-put,  569,  570 

Pressure  wires,  use  of,  502 

Radiation,  electromagnetic,  6 
,  solar,  26 


Reactance,  137 

Rectifier,  electrolytic,  288,  289 
Resistance  of  wires  to  alternating 
current,  456,  457 

,  unit  of,  20 

Resonance,  150,  151,  152,  451 

,  apparatus  for  showing,  151 

,  calculation  of,  451,  452, 453, 

454,  455 

Rotary  converter,  282 

converter,  examples  of, 

283,  284 

converter,  principles  of, 

283 

converter,  ratio  of  trans- 
formation in,  286 

— • transformers,  advantages 

and  disadvantages  of,  286,  287 

Secondary  mains,  voltage  of,  507, 
508 

Shafting,  losses  in,  64 

Sine  wave,  124 

wave,  variations  from,  125, 

126 

Southern  California  transmission 
lines,  607,  608,  609,  610,  6n,  612 

Span,  permissible  lengths  of,  475, 
476 

Sparking  distance  of  various  volt- 
ages, 494 

Star  connection  of  three-phase 
circuits,  180 

Strains,  electrostatic  and  electro- 
magnetic, 10 

Sub-stations,  arc  lighting  in,  510, 
511,  512,  513,  514 

,  arrangement  of,  538,  539 

,  location  of,  536,  537,  538 

« ,  motor  service  from,  539 

Switch  boards,  414,  415 

Switches,  high  voltage,  616,  617 

Synchronous  induction  motor,  232, 

233 
motors,  209,  210 

motors,  current  variation 

in,  213,  214 

motors,  effect  of  excitation 

on  power  factor  of,  215,  216 

motors,  lag  of  current  in, 

213 

motors,  methods  of  start- 
ing, 221,  222 

motor,    monophase    self- 


starting,  222,  223 

motors,     organization 

currents  in,  209,  210 

motors,    polyphase,    : 

224 


of 


632 


INDEX. 


Synchronous  motors,  power  fac- 
tors of,  in  practice,  218,  219 
motors,  starting,  211,  212 

Three  phase  circuits,  178 

circuits,    connections    of, 

180,  181 
circuits,  saving  of  copper 

in,  182 

Three- wire  system,  no,  in 
Tides,  utilization  of,  26 
Transformers,    construction     of, 

191,  192 

,  cooling  of,  195,  196 

,  efficiency  of,  194,  197 

,  losses  in,  192,  193 

,  polyphase,  198,  199,  200 

,  principle  of,  191 

,  small,  efficiency  curve  of, 

435 

,  small  vs.  large,  436 

,    sub-station,     installation 

of,  353,  354 
Transmission  lines,   location  of, 

467,  468,  469 
lines,  practical  calculation 

of,  460,  461,  462,  463, 464,  465, 466 
Transformer  as  booster,  205 
Turbine,  steam,  De  Laval,  314,  315 
,    steam  efficiency  of,   315, 

319 
,  steam,  Parsons,  316,  317, 

318,  319.  320 

,  steam,  principles  of,  314 

Turbines,   efficiency  of,  338,  339, 
340,  341 

,  impulse,  327,  328 

,  pressure,  326,  327,  335,  336 

,  principles  of,  325,  326 

,  regulation  of,  339,  345 

,  regulation  of,  under  vary- 
ing heads,  342,  343,  344 
,   types  of  American,   328, 

329,  330,  331,  332 

Two-phase-three-phase    transfor- 
mation, 203,  204 

Units,  C.  G.  S.,  19 
,  electrical,  19 

Valve  gear,  dependent,  295 

,  independent,  294 

Volt,  20 

Voltage    regulation,     automatic, 

545,  546,  547,  548 
Voltage    regulators,    sub-station, 

548,  5.49,  550,  551,  552 
Voltmeters,  578,  579 
,  recording,  580,  581 


Water    power,    classification    of, 
361,  362 

power,  commercial  devel- 
opment of,  383,  384 

power,  distribution  of,  24 

power,      estimate     for     a 

cheap  plant,  567,  568 

power,  estimate  for  a  typi- 


by 


cal  plant,  565,  566 

power,    measurement 

miner's  inches,  365,  366 

power    measurement    by 

soundings,  366,  367 

power,    measurement    of 

flow  in,  363 

power,  methods  of  develop- 
ing, 372,  373,  374,  375,  376,  377, 
378,  379,  38o 

power,   relation    between 

flow  and  HP  in,  569 

power,   relation    between 


rainfall  and  flow  in,  368,  369 
power,  storage  of,  370,  371, 

382 

power,  treatment  of  vari- 
able supply,  382,  383 
power,  weir  measurement 

of,  363,  364,  365 
power  with  supplementary 

steam  power,  383,  384 

wheels,  324 

Watt,  21 

,   relation  of,   to  electrical 

energy,  21 
Wattmeters,  connections  of,  583, 

585,  5.86,  587,  588,589 

,  indicating,  579,  580 

,  integrating,  582,  583 

,  inspecting    and     testing, 

59°,  59i 
Wave  shape,  practical  aspects  of, 

264,  265 

Wind,  utilization  of,  25 
Windings,  armature,  polyodontal, 

174 
Wire  for  lines,  mechanical  strains 

upon,  471,  472 
for    lines,    properties    of, 

470 

formulae  and  tables,  420 

,  stresses  due  to  load  upon, 

473,  48i 
,  stresses  upon,  due  to  wind, 

474 

,  stresses  upon,  from  tem- 
perature, 472,  473 
,  stresses  upon  suspended, 


472 
Wires,  insulated,  443 


YD  07503 


989189 


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